The history of Christianity begins with the ministry of Jesus, a Jewish teacher and healer who was crucified and died c. AD 30–33 in Jerusalem in the Roman province of Judea. Afterwards, his followers, a set of apocalyptic Jews, proclaimed him risen from the dead. Christianity began as a Jewish sect and remained so for centuries in some locations, diverging gradually from Judaism over doctrinal, social and historical differences. In spite of the persecution of Christians in the Roman Empire, the faith spread as a grassroots movement that became established by the third-century both in and outside the empire. New Testament texts were written and church government was loosely organized in its first centuries, though the biblical canon did not become official until 382.

Constantine the Great was the first Roman Emperor to declare himself a Christian. In 313, he issued the Edict of Milan expressing tolerance for all religions. He did not make Christianity the state religion, but he did provide crucial support. Constantine called the first of seven ecumenical councils. In the fourth century, Eastern and Western Christianity had already begun to diverge. Between 600 and 750, the constant need to defend itself in war turned the Eastern Roman Empire into the independent polity of Byzantium. Missionary activities spread Christianity across western Europe. Monks and nuns were prominent in establishing a Christendom that influenced every aspect of medieval life.

From the ninth century into the twelfth, politicization and Christianization went hand-in-hand in developing East-Central Europe. Byzantine Christianity influenced the church, culture, language, literacy, and literature of Slavic countries and Russia. During the High Middle Ages, Eastern and Western Christianity had grown far enough apart that differences led to the East–West Schism of 1054. Temporary reunion was not achieved until the year before the fall of Constantinople in 1453. The fall of the Byzantine Empire ended the institutional Christian Church in the East as established under Constantine, though it survived in an altered form.

Various catastrophic circumstances, combined with a growing criticism of the Catholic Church in the 1300–1500s, led to the Protestant Reformation and its related reform movements. Reform, and the Catholic Counter-Reformation, were followed by the European wars of religion, the development of modern political concepts of tolerance, and the Age of Enlightenment. Christianity also influenced the New World.

After World War II, Christianity faced many challenges. Traditional Christianity has declined in the West, while new forms have developed and expanded worldwide. Today, there are more than two billion Christians worldwide. Within the last century, the centre of growth has shifted from West to East and from the North to the Global South making Christianity a truly global religion in the twenty-first century.

Christianity emerged in the Roman province of Judea during the first-century. The first Christian communities were predominantly Jewish. The religious, social, and political climate in Judea was extremely diverse and characterized by turmoil. Judaism itself included numerous religious and political movements. The prophecy and poetry of one such movement, Jewish messianism, promised a future anointed leader descended from King David to restore the Israelite Kingdom of God and replace the foreign rulers. As indicated by their texts and organisation, the first Christians saw one man, Jesus, as that promised Messiah.

Early Christianity began with the ministry of Jesus (c. 27–30). Jesus' existence and his crucifixion are historically well attested. Jesus saw his identity and mission, and that of his followers, in light of the kingdom of God and the prophetic tradition of Israel. His followers believed God's spirit was incarnated (embodied) in Jesus and that after his crucifixion, he rose from the dead. The Christian church established incarnation and resurrection as its first doctrines, with baptism and the celebration of the Eucharist meal (Jesus's Last Supper) as its two primary rituals.

Early Christians first gathered into small groups inside private homes, where the typical setting for worship was the communal meal. Presbyters or bishops oversaw the economic requirements of the meal, alongside charitable distributions, and any ceremonial role they took was initially connected to this more prosaic role. Bishops soon began forming a Christian elite.

Beginning with less than 1000 people, Christianity grew to around one hundred small household churches consisting of an average of seventy members each, by the year 100. It spread through the Jewish diaspora along the trade and travel routes followed by merchants, soldiers, and migrating tribes. It had reached Ancient Greece, and probably Alexandria in the first century, leading to the development of Coptic Christianity. Paul was one of the Apostles who spread Christianity in the first century, making at least three missionary journeys and founding numerous churches in Asia Minor; Christianity in Antioch is mentioned in his epistles.

The Jews of Alexandria had produced a Greek translation of their Hebrew Bible between the third and first centuries BC which the apostles and early Christians used. Christian writings in Koine Greek, including the four gospels (the accounts of Jesus's ministry), letters of Paul, and letters attributed to other early Christian leaders, were written in the first-century and had considerable authority even in the formative period. The letters of the Apostle Paul sent to the early Christian communities were circulating in collected form by the end of the first-century.

At the Council of Jerusalem, likely held in 49 or 50, the Jerusalem church gathered to address whether the increasing numbers of non-Jews who were joining the movement needed to follow Jewish law. The destruction of Jerusalem by the Romans in 70, alongside the development of Rabbinic Judaism and disagreements about Jewish law or insurrections against Rome, contributed to the divergence of Jewish and Christian practices. Nevertheless, Jewish Christianity remained influential in Palestine, Syria, and Asia Minor into the second and third centuries.

Christianity achieved critical mass in the years between 150 and 250, when it grew from fewer than 50,000 adherents to over a million. By the third century, it had reached as far as Roman Britain in the West, into North Africa, and into the Balkans in the East. A more formal Church structure grew over the second and third centuries AD, but at different times in different locations. Bishops rose in power and influence, beginning to preside over larger areas with multiple churches.

Early Christianity was open to everyone. Baptism was free, and there were no fees, which made Christianity a substantially cheaper form of worship compared with the traditional Roman models. The religion's inclusivity extended to women, who made up significant numbers of Christianity's earliest members. Women could attain greater social freedom through religious activities than customary Roman expectations permitted. Women were prominent in early documentary sources such as church rolls or the Pauline epistles and ubiquitous in early Christian art, while much early anti-Christian criticism was linked to "female initiative", indicating their prominent role in the movement.

A key characteristic of Early Christianity was its unique type of exclusivity. Believing was the crucial and defining characteristic of membership— believers were separated from the "unbelievers" by a strong social boundary. This exclusivity gave Christianity the powerful psychological attraction of elitism.

A set of early Christian writings similar to the current New Testament existed by the early third century. The four gospels and the letters of Paul were generally regarded as authoritative, but other writings, such as the Book of Revelation and the epistles to the Hebrews, James, and John, were assigned different degrees of authority. Gnostic texts challenged the physical nature of Jesus, Montanism suggested that the apostles could be superseded, and Monarchianism emphasized the unity of God over the Trinity. In the face of such diversity, unity was provided by the shared scriptures and bishops who established common ground.

The Ante-Nicene period included increasing but sporadic persecution from Roman authorities, and the rise of Christian sects, cults, and movements. The first persecution by a Roman emperor was under Nero, probably in 64 AD, when it is conjectured the Apostles Peter and Paul were killed. In the 250s, the emperors Decius and Valerian made it a capital offense to refuse to make sacrifices to Roman gods, resulting in widespread persecution of Christians. Official persecution reached its height under Diocletian in 303–311. Authorities in the Sasanian Empire also periodically persecuted Christians.

There are few early records of early Christian art. The likely oldest forms—frescoes or statues—emerged in funerary environments c. 200. It typically fused Graeco-Roman style and Christian symbolism: the most common image was Jesus as the good shepherd.

The emperor Constantine, a self-declared Christian, issued the 313 Edict of Milan expressing tolerance for all religions. He supported Christianity, giving bishops judicial power and legally establishing them as equal to polytheistic priests. He devoted personal and public funds to building churches and endowed them with wealth for their clergy and upkeep. As a result, Christian art and literature blossomed, and churches were built in the vast majority of Roman cities by the end of the century.

Imperial laws aimed at suppressing sacrifice and magic were also issued. Blood sacrifice had been a central rite of virtually all religious groups in the pre-Christian Mediterranean, but it disappeared by the end of the fourth century. Occasional hostile actions occurred, but pagan-Christian religious violence was not a general phenomenon. There was no legislation forcing the conversion of pagans before the 500s. Polytheism remained widespread for centuries, as late as the 800s in Greece.

Jews and Christians were originally both religious minorities who claimed the same inheritance, and thus directly competed. The theologian Augustine of Hippo argued that the Jews were not to be killed or forcibly converted, but instead left alone to practice Judaism because they preserved the teachings of the Old Testament and were "living witnesses" of the New. As a result, most Jews in Europe lived relatively peacefully alongside Christians until the 1200s. The theology of supersessionism claimed that Christians had displaced the Jews as God's chosen people; many attribute the emergence of antisemitism to this concept, while others distinguish between them.

Constantine and his successors attempted to fit the Church into their political program. Church leaders disagreed, arguing that religious authority was separate from state authority. For most of Late Antiquity, the popes—the successors to Saint Peter as bishop of Rome— had limited influence, but this began to change as eastern patriarchs increasingly looked to the Pope to resolve disagreements.

Christian monasticism originated before the fourth century in Syria, deriving from the asceticism already intrinsic to Christianity. Monastic communities were associated with the urban holy places in Palestine, which became a center of pilgrimage, Cappadocia, Italy, Gaul and Roman North Africa. In 358, Basil the Great founded a monastic community in Caesarea that developed a transformative health care system, which allowed the sick to be cared for in a special building, which became a model for public hospitals for many centuries.

Christianity continued to grow rapidly, both westwards and eastwards: In the fourth century the percentage of Christians was as high in the Sasanian Empire as the Roman Empire. Even as the Huns, Ostrogoths, Visigoths, and Vandals caused chaos in the Roman Empire in the fourth and fifth centuries, many converted to Christianity. Although the religion soon reached as far east as Afghanistan and as far south as Soqotra, Christian institutions in Asia or East Africa never developed the intellectual or socio-political power of the European churches.

Armenia became the first state to adopt Christianity as its religion in 301. It was followed by others in the Caucasus, such as Albania, and Ethiopia and Eritrea in Africa. Christianity, a minority faith in Britain since the second century, began to be displaced by Anglo-Saxon paganism in the fifth century; this process began reversing after the Gregorian mission of 597. Missionaries also began to convert the Irish in the early fifth century.

The Church was seen by its supporters as a universal church based on belief. Ancient authors identified practices and doctrines which differed from "correct belief", as defined by orthodox tradition, as heresy. The number of laws directed at heresy indicate it was a much higher priority than paganism for Christians of this period. Arianism, the great heresy of this period, argued against the traditional concept of the trinity. The First Council of Nicaea in 325 attempted to resolve the controversy with a statement of orthodoxy called the Nicene Creed.

Christian scripture was formalised as the New Testament by the fourth century, distinguishing it from the Hebrew Old Testament. Despite agreement on these texts, differences between Eastern and Western churches were becoming evident. Latin was used by the west but not the east, where Greek, Syrian, and other languages were used. The West condemned Roman culture as sinful and resisted state control, whereas the east harmonised with Greek culture and aimed for unanimity between church and state. There were also tensions surrounding the marriage of clerics, which continued in the east but was forbidden in the west. The East advocated for sharing the government of the church between five leaders arguing that the patriarchs of Constantinople, Alexandria, Antioch, and Jerusalem were equal to Rome. Rome asserted papal primacy.

Controversies over how Jesus' human and divine natures co-existed, led to a series of ecumenical councils: the Third in 431, the Fourth in 451, the Fifth in 583, and the Sixth in 680–681. The Fourth Council's assertion—that two separate natures of Christ form one ontological entity—was rejected by the Armenian, Assyrian, and Egyptian churches, who split from the rest of Christianity and combined into Oriental Orthodoxy.

Constantine's sponsorship produced a burst of Christian art and architecture. The basilica, a type of Roman municipal courthall, became the model for all later Christian architecture. This led to an age of frescoes, mosaics, statues and paintings which blended classical and Christian styles. Similarly, a hybrid form of poetry written in classical styles with Christian concepts emerged.

The codex, the ancestor of modern books, was already used by first century Christians, but the Egyptian church likely invented the papyrus codex during the next decades. In the late fourth century, Jerome was commissioned to translate the Greek biblical texts into the Latin language; this translation was called the Vulgate. Church fathers of this period, such as Augustine of Hippo, John Chrysostom, Gregory of Nyssa, Athanasius of Alexandria, Basil of Caesarea, Gregory of Nazianzus, Cyril of Alexandria, and Ambrose of Milan, wrote huge numbers of works, contributing to a golden age of writing.

While Christianity still saw itself as universal, three distinct cultures emerged between 600 and 750: Germanic Europe in the west, and that of the Byzantine Empire and Islamic civilization to the east.

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In this age, a great deal of diversity existed, yet the concept of Christendom was also pervasive and unifying. Medieval writers and ordinary folk used the term to identify themselves, their religious culture, and even their civilization. Mixed within and at the edges of this largely Christian world, barbarian invasion, deportation, and neglect also produced large "unchurched" populations. In these areas, Christianity was often one religion among many and could combine with aspects of local paganism. Early medieval religious culture included "worldliness and devotion, prayer and superstition", but its inner dynamic sprang from belief in universal Christendom.

The means and methods of teaching a mostly illiterate populace included mystery plays (which had developed out of the mass), wall paintings, vernacular sermons and treatises, and saints' lives in epic form. Christian motifs could function in non-Christian ways, while practices of non-Christian origin became endowed with Christian meaning. From the sixth to the eighth centuries, most schools were monastery-based.

Throughout this period, a symbiotic relationship existed between ecclesiastical institutions and civil governments. Canon law and secular law were connected and often overlapped. Canon laws, created by councils, kings, bishops, and lay assemblies, enabled the church to sustain itself as an institution and wield social authority with the laity. In the East, Roman law remained the standard. The West was a world of relatively weak states, endowed aristocracies, and peasant communities that could no longer use law from a past empire to support the church. Instead, the Western church adopted a feudalistic oath of loyalty which became a condition of consecration.

The oath of loyalty between men and their king created a new model of consecrated kingship. Janet Nelson writes that:

This rite has a continuous history in both Anglo-Saxon England and Francia from the eighth-century onward, with further refinements in the ninth and tenth. It is, among other things, a remarkable application of law by early medieval churchmen in the West, to which the East offers no parallel.

Despite the fragmentation and end of the Western Roman Empire in the fifth century, the Roman Senate remained influential, playing an increasing role in church politics. In Gaul, the Frankish king Clovis I converted to Christianity; his kingdom, the dominant polity in the West, was converted over the next centuries while much of Western Europe remained impoverished and politically fragmented. In the eighth century, Clovis' descendant Charlemagne began the Carolingian Renaissance, a period of intellectual and cultural revival which continued under his successors. Christianity became dominant in England during the seventh century, while suppression of Germanic paganism began. There are no recorded heathen kings after 954.

Popes led the sixth-century response to the invasion of northern Italy by the Lombards (569) producing an increase in papal autonomy and prestige. By the time Pope Gregory I succeeded to the papacy in 590, the claim of Rome's supremacy as stemming from Peter can be seen as established even though large sections of both the Western and Eastern churches remained unconvinced they should be submissive to the Roman See. Papal power rose as competition within the church increasingly led people to Rome to resolve disagreements. The growing presence and involvement of the aristocracy in the papal bureaucracy, an increase in papal land-holdings from the second half of the sixth into the seventh-century, combined with changes in their administration that brought an increase in wealth, gradually shifted popes from being beneficiaries of patronage to becoming patrons themselves. William IX, Duke of Aquitaine, and other powerful lay founders of monasteries, placed their institutions under the protection of the papacy in the tenth-century thereby facilitating another rise in papal power.

In the Eastern Empire, culture blossomed during the reign of Emperor Justinian I (527–565). Justinian made donations to the church, established foundations, and watched over church property. He supported the rights of bishops, priests, abbots, and monastic life. In 563, after earthquakes destroyed it, Justinian rebuilt the Hagia Sophia using ten thousand workers and 40,000 pounds of silver, covering the dome in gold. He integrated Christian concepts with Roman law, creating the Code of Justinian. This code became an essential part of the Corpus Juris Civilis, which remains the basis of civil law in many modern states.

Justinian's religious policies reflected his conviction that the unity of the Empire presupposed unity of faith. He persecuted pagans and religious minorities and purged the governmental bureaucracy of those who disagreed with him. He regulated everything in religion and law, even interfering in papal elections. Manichaeism rose in southern Mesopotamia in the third-century and expanded from the fourth to sixth centuries in almost all parts of the Roman Empire, especially Syria, Mesopotamia, Egypt, North Africa and Italy. Severe persecution instigated by emperor Justinian ended them.

In the early 600s, Christianity extended around the Mediterranean, across much of Europe into Spain and Britain, East to the edge of Central Asia as far as Zerang and Qandahar in modern Afghanistan, and into the Sassanian Persian Empire, with Christian churches concentrated in northern Iraq in the foothills of the Zagros, and in the trading posts of the Persian Gulf. A vibrant Asian Christianity with nineteen metropolitans (and eighty-five bishops), centred on Seleucia (just south of Baghdad), flourished in the eighth-century. Two kinds of Christian communities had formed in Syria, Egypt, Persia, and Armenia: urban churches which upheld the Council of Chalcedon, and Nestorian churches from the desert monasteries.

Born in the seventh century, Islamic civilization, in a series of Arabic military campaigns between 632 and 750, and diplomacy, conquered much of Syria, Mesopotamia, Egypt, Persia, North Africa, and Spain. By 635, upper-class Christian refugees had moved further east to China at Hsian-fu. Inferior legal status and persecution of non-Muslims eventually devastated the Chalcedonian churches in the cities. The monastic background of the Nestorians made their churches more remote, making them the most able to survive and cultivate new traditions.

During the sixth-century, monasticism flourished nearly everywhere Christianity existed. It developed somewhat differently in each region and by 600 there was great diversity even though monasteries shared basic elements. Monastics followed a discipline of devotional practices aimed at cultivating an awareness of God which generally included formalized prayer, memorization of scripture, celibacy, fasting, manual labor, and almsgiving.

Monasteries became more and more organized from 600 to 1100. The formation of these organized bodies of believers gradually carved out social spaces with authority separate from political and familial authority, thereby revolutionizing social history. Medieval monasteries provided orphanages, hostels (inns) for travelers, distributed food during famine, and regularly provided food to the poor. They supported literacy, ran schools, and copied and preserved ancient texts in their scriptoria and libraries. They practiced classical craft and artistic skills, while maintaining an intellectual and spiritual culture that developed and taught new skills and technologies. In the early sixth-century, Benedict of Nursia wrote the Rule of Saint Benedict which would become the most common monastic rule, the starting point for others, and would impact politics and law throughout the Middle Ages. Medical practice was highly important, and medieval monasteries are best known for their contributions to medical care.

Dedicated monks merged Germanic, Celtic, and classical traditions to create "illuminated" psalters, collections of the Psalms, the gospels, and copies of the Bible. First, using geometric designs, foliage, mythical animals, and biblical characters, the illustrations became more realistic in the Carolingian Renaissance.

In the 720s, the Byzantine Emperor Leo banned the pictorial representation of Christ, saints, and biblical scenes, destroying much of early art history. The West condemned Leo's iconoclasm. By the tenth and early eleventh centuries, Byzantine culture began to recover.

The Middle Ages saw momentous changes in the economic and political structures of Europe and the birth of institutions that became fundamental to Christianity. Many Roman Catholic fundamentals – "the meaning of the sacraments, the just price and reward for labour, the terms of Christian marriage, the nature of clerical celibacy and the appropriate lifestyle for priests" – were conceived in the twelfth-century. Heresy was defined with new precision. Purgatory became an official doctrine, and in 1215, confession became required for all. Veneration of the Virgin Mary rose, became a central aspect of the age, and led to the invention of the rosary toward the end of this period.

Membership in Christendom began with baptism at birth. Members were required to have a rudimentary knowledge of the Apostles' Creed and the Lord's Prayer. From peasant to pope, all were required to rest on Sunday and feast days, attend mass, fast at specified times, take communion at Easter, pay various fees, tithes, and alms for the needy, and receive last rites at death.

After 1054, the term Christendom did not include the Eastern churches. The Byzantine East and the Catholic West had long had many irreconcilable differences. Along with a general lack of charity and respect on both sides, there were also many cultural and linguistic differences, geographical separation, and geopolitical disagreements. In 1054, this produced the East–West Schism, also known as the "Great Schism", which separated the Church into Western Catholicism and Eastern Orthodoxy.

In the early part of this era, Viking raids and civil disorder destroyed many churches and monasteries, inadvertently leading to reform and improvement. Patrons competed against each other in rebuilding, adding to, and improving replacements so that "by the mid-eleventh century, a wealthy, unified, better-organized, better-educated, more spiritually sensitive Latin Church" resulted.

Gregorian Reform (1050–1080) increased papal power, centralized church authority, and reinforced clerical celibacy. Under Hugh of Cluny (1049–1109), the Abbey of Cluny became the leading centre of reform in Western monasticism from the eleventh into the early twelfth-century. The Cistercian movement, a second wave of reform after 1098, also became a primary force of technological advancement and its spread in medieval Europe contributing to economic growth.

Beginning in the twelfth-century, Mendicant orders (Franciscans and Dominicans) embraced a reformation in understanding a monk's calling as a charge to go out and actively reform the world. The papacy recruited and licensed this new type of charismatic preacher whose audience was no longer the people of a parish or a city, but whole regions.

The Investiture controversy, which began in the Holy Roman Empire in 1078, was specifically a conflict between the Holy Roman Emperor Henry IV and Pope Gregory VII (1073–1085) over who had the right to invest a bishop or abbot into a paying job. More generally, it was over control of the church and its revenues.

Bishoprics were lifetime appointments, so a king could better control their powers and revenues than those of hereditary noblemen. He could place an aspirant of his choice, or leave the post vacant and collect the revenues himself, (theoretically in trust for a future bishop), or he could give a bishopric to compensate a helpful noble. For the church, ending this would better separate church from state, help with reform, and provide better pastoral care. For the civil authorities, ending lay investiture meant the power of the Holy Roman Emperor and the European nobility would be reduced.

Gregory recorded a series of statements asserting that the church must be the higher of the two powers of church and state and that the church must no longer be treated as a servant to the state. Disobedience to the Pope became equated with heresy. The Dictatus Papae of 1075 declared the pope alone could invest bishops. Henry IV rejected the decree. This led to his excommunication, which contributed to a ducal revolt, that led to a civil war: the Great Saxon Revolt. Eventually, Henry received absolution. The conflict lasted five decades. A similar controversy occurred in England.

In 1081 Alexios I Komnenos asked Pope Urban for help with the Seljuk Turks. Urban responded (1095) with an appeal to European Christians to "go to the aid of their brethren in the Holy Land". Urban's message had tremendous popular appeal, and there was much enthusiasm supporting it. It was new and novel and tapped into powerful aspects of folk religion. Voluntary poverty and its renunciation of self-will, along with a longing for the genuine "apostolic life", flourished in the late eleventh and twelfth centuries connecting pilgrimage, charity, remission of sins, and a willingness to fight.

Crusading involved the church in certain paradoxes: Gregorian reform was grounded in distancing spirituality from the secular and the political, while crusade made the church dependent upon financing from aristocrats and kings for the most political of all activities: war.

Crusades led to the development of national identities in European nations, increased division with the East, and produced cultural change. Hotly debated by historians, the single most important contribution of the Crusades to Christian history was, possibly, the invention of the indulgence.

The medieval papacy gained authority in every domain of life. Under Pope Gregory VII, the scope of canon law was extended, and the church became a more imposing institution, consolidating its territory and establishing a bureaucracy. State administrations were also centralizing and developing bureaucracies. Competition (and emulation) between them developed into a battlefield of ideas and personalities as both church and state claimed legal jurisdiction and the right to collect taxes from the same people.

Canon law became a large and highly complex system of laws that omitted Christianity's earlier principles of inclusivity.

Popes from 1159 to 1303 were predominantly lawyers, not theologians. New networks and new agencies were often manifested as legal services, and over it all watched an increasingly centralized and proactive church government. The papacy's power and influence gradually came to resemble that of the monarchs of its day.

Throughout Christian Europe, church and civic rulers made efforts to support coherence and order. Moral misbehavior and heresy, by the folk and clerics, were prosecuted by inquisitorial courts that were composed of both church and civil authorities. Bishops were in charge. The Medieval Inquisition includes the Episcopal Inquisition (1184–1230) and the Papal Inquisition (1230s–1240s), though these courts had no actual joint leadership or organization. Created as needed, they were not permanent institutions but were limited to specific times and places. The Medieval Inquisition brought somewhere between 8,000 and 40,000 people to interrogation and sentence. Death sentences were a relatively rare occurrence. The penalty imposed most often by Medieval Inquisitorial courts was an act of penance which could include public confession. Medieval inquisitors did not possess absolute power, nor were they universally supported. Riots and public opposition formed as inquisition became stridently contested both in and outside the Church. The fourth Lateran council empowered inquisitors to search out moral and religious "crimes" even when there was no accuser. In theory, this granted them extraordinary powers. In practice, without local secular support, their task became so overwhelmingly difficult that inquisitors themselves became endangered.

In 1209, Pope Innocent III and the King of France, Philip Augustus, began a military campaign to eliminate the Albigensian heresy known as Catharism. Once begun, the campaign quickly took a political turn. The king's army seized and occupied strategic lands of nobles who had not supported the heretics, but had been in the good graces of the Church. Throughout the campaign, Innocent vacillated, sometimes taking the side favouring crusade, then siding against it and calling for its end. It did not end until 1229 when the region was brought under the rule of the French king, creating southern France, while Catharism continued for another hundred years (until 1350).

Traditionally, schools had been attached to monasteries, though Cathedral schools were established in the early Middle Ages, and independent schools arose in some of the larger cities. For most folk, learning began at home, then continued in the parish where they had been born and were associated with for the rest of their lives. The clergy, and the laity, became "more literate, more worldly, and more self-assertive" and they did not always agree with the hierarchy.

Between 1150 and 1200, intrepid monks traveled to formerly Muslim locations in Sicily and Spain. Fleeing Muslims had abandoned their libraries, and among the treasure trove of books, the searchers found the works of Aristotle, Euclid and more. Adapting Aristotelian logical reasoning and Christian faith created a revolution in thinking called scholasticism which elevated reason and reconciled it with faith. This led to a twelfth-century renaissance which included the revival of the scientific study of natural phenomena. Historians of science see this as the beginning of what led to modern science and the scientific revolution in the West.

From the 1100s, Western universities, the first institutions of higher education since the sixth century, were formed as self-governing corporations chartered by popes and kings. Bologna, Oxford and Paris were among the earliest (c. 1150). Divided into faculties which specialized in law, medicine, theology or liberal arts, each held quodlibeta (free-for-all) theological debates amongst faculty and students and awarded degrees. Canon and civil law began to be professionalized.

This was a period of enormous creativity characterised by an imposing public Christian art full of light, colour, and rhythm. Romanesque style using Roman features with Christian influences, emerged in Europe between 1000 and 1200 as an aspect of the monastic revivals, especially the Cluniacs. It was used primarily in architecture but also produced statuary, paintings, and illustrated manuscripts. Between 1137 and 1144 the Gothic style, with ribbed vaults and flying buttresses, such as those found in Notre Dame and the cathedral at Amiens, was invented. The monk Guido of Arezzo modernized musical notation, invented the music staff of lines and spaces, and began the naming of musical notes making modern music possible.

By the end of the eleventh-century, Christianity was in full retreat in Mesopotamia and inner Iran. Some Christian communities further to the east continued to exist.

The Christian churches in Egypt, Syria, and Iraq became subject to fervently Muslim militaristic regimes. Christians were dhimma. This cultural status guaranteed Christian's rights of protection but discriminated against them through legal inferiority. Various Christian communities adopted different strategies for preserving their identity while accommodating their rulers. Some withdrew from interaction, others converted, while some sought outside help.

Christianization of Scandinavia (Sweden, Norway, and Denmark) occurred in two stages. In the first stage, missionaries arrived on their own, without secular support, in the ninth-century. Next, a secular ruler would take charge of Christianization in their territory. This stage ended once a defined and organized ecclesiastical network was established. By 1350, Scandinavia was an integral part of Western Christendom.

When the Second Crusade was called after Edessa fell, the nobles in Eastern Europe refused to go. The Balts, the last major polytheistic population in Europe, had been raiding surrounding countries for several centuries, and subduing them was what mattered most to the Eastern-European nobles. In 1147, Eugenius' Divina dispensatione gave eastern nobility indulgences for the first of the Baltic wars (1147–1316). The Northern Crusades followed intermittently, with and without papal support, from 1147 to 1316. Priests and clerics developed a pragmatic acceptance of the forced conversions perpetrated by the nobles, despite the continued theological emphasis on voluntary conversion.

In the twelfth-century, Byzantium was weakened from repeated invasion, and its territorial frontiers became nebulous, but economically and spiritually, the core of the Byzantine Empire was never more prosperous. Three powerful groups – Seljuk Turks from the east, Almoravids from West Africa, and Crusaders from Europe – began to change the politics, culture, and religious configurations of Byzantium, Islamic civilization, and the European West.

From the 950s to the 980s, polytheism among the Kievan Rus declined, while many social and economic changes fostered the spread of the new religious ideology known as Christianity. The event associated with the conversion of the Rus' has traditionally been the baptism of Vladimir of Kiev in 989.

The new Christian religious structure was imposed by the state's rulers. The Rus' dukes maintained control of the church which was financially dependent upon them. While monasticism was the dominant form of piety, Christianity permeated daily life, for both peasants and elites, who identified themselves as Christian while keeping many pre-Christian practices.

Beginning under emperor Basil I (r. 867–886), Byzantine Christianity was instrumental in forming what would become Eastern Europe. Serbia, Alania (modern Iran), Russia and Armenia were nascent Christian states by the early eleventh-century. Romania, Bulgaria, Poland, Hungary and Croatia soon followed.

Saints Cyril and Methodius translated the Bible, developing the first Slavic written script and the Cyrillic alphabet in the process. This became the educational foundation for all Slavic nations and influenced the spiritual, religious, literary, and cultural development of the entire region for generations.

A turning point in Jewish-Christian relations took place in June 1239 when the Talmud was put "on trial" by Gregory IX (1237–1241) in a French court over contents that mocked the central figures of Christianity. This resulted in Talmudic Judaism being seen as so different from biblical Judaism that old Augustinian obligations to leave the Jews alone no longer applied. As townfolk gained a measure of political power, they became one of Jewry's greatest enemies, charging Jews with blood libel, deicide, ritual murder, poisoning wells and causing the plague, and various other crimes. Although subordinate to religious, economic, and social themes, racial concepts also reinforced hostility.

Jews had often acted as financial agents for the lords providing them loans with interest while being exempt from taxes and other financial laws themselves. This attracted jealousy and resentment. Emicho of Leiningen massacred Jews in Germany in search of supplies, loot, and protection money. The York massacre of 1190 also appears to have had its origins in a conspiracy by local leaders to liquidate their debts along with their creditors.

The many calamities of the "long fourteenth-century" – plague, famine, multiple wars, social unrest, urban riots, peasant revolts, and renegade feudal armies – led folk to believe the end of the world prophesied in the New Testament book of Revelation was imminent. This belief ran throughout society and became intertwined with anticlerical and anti-papal sentiments. While there is scholarly disagreement over using "anticlericalism" to describe the pre-Reformation Middle Ages, it is generally used by scholars. Defined simply as medieval criticism of the clergy, anticlericalism in the Middle Ages ranges from criticism of individual clergy to criticism of the entire church.

Clergy were privileged. They had secular and economic power and lay people depended on them to administer the sacraments. The combination of privilege and dependence caused resentment. The ideal of how clergy should be and claims of deviations (which might include simony, gluttony, sexual immorality, and violence) were prominent in all the anticlerical movements.

Anticlerical sentiment, by lay people and clerics, appeared in heretical movements, internal reform movements, and popular writings, both secular and religious. A serious decline in faith in the clergy by the end of the 15th century identifies this era as a period of "anticlerical revolution". This anticlericalism was an integral part of medieval life.

Throughout the Late Middle Ages, the church faced powerful challenges and vigorous political confrontations. Clerics voiced many of the criticisms made between 1100 and 1520 condemning abuses, and seeking a more spiritual, less worldly clergy. However, there is a constancy of complaint in the historical record that indicates most attempts at reform between 1300 and 1500 failed.

John Wycliffe (1320–1384), an English scholastic philosopher and theologian, attended the Council of Constance (1414–1418) and urged the Church to give up its property (which produced much of the Church's wealth), to once again embrace poverty and simplicity, to stop being subservient to the state and its politics, and to deny papal authority. He was accused of heresy, convicted, and sentenced to death, but died before implementation.

Jan Hus (1369–1415), an evangelical Czech based in Prague, was influenced by Wycliffe and spoke out against what he saw as abuses and corruption in the Catholic Church. He was also accused of heresy and condemned to death. Hus became a powerful symbol of Czech nationalism and the impetus for the Bohemian (aka the Czech) Reformation.

During the Late Middle Ages, groups of laymen and non-ordained secular clerics sought a more sincere spiritual life. A vernacular religious culture for the laity arose. The new devotion worked toward the ideal of a pious society of ordinary non-ordained people.

In 1309, Pope Clement V moved to Avignon in southern France in search of relief from Rome's factional politics. The Avignon Papacy consisted of seven popes whose residence there produced unintended consequences for the papacy. The move away from the "seat of Peter" caused great indignation throughout the church and cost popes prestige and power. Papal power stopped rising.

Pope Gregory XI returned to Rome in 1377. After Gregory's death, the papal conclave met in 1378, in Rome, and elected an Italian Urban VI to succeed Gregory. The French cardinals did not approve, so they held a second conclave electing Robert of Geneva instead, giving the church two popes. This began the Western Schism.

For the next thirty years the Church had two popes, then in 1409, the Pisan council called for the resignation of both popes, electing a third to replace them. Both Popes refused to resign, leaving the Church with three popes. Five years later, Sigismund the Holy Roman Emperor (1368–1437) pressed Pope John XXIII to call the Council of Constance and depose all three popes. In 1417, the council elected Pope Martin V in their place.

During the European Renaissance of the 15th and 16th centuries, the Church was a leading patron of art and architecture, directly commissioning many individual works and supporting many artists such as Michelangelo, Brunelleschi, Bramante, Raphael, Fra Angelico, Donatello, and Leonardo da Vinci.

Scholars revealed the Donation of Constantine as a forgery.

Literature was impacted by Dutch scholar Desiderius Erasmus (1466–1536), an outstanding figure of Christian humanism in the sixteenth century. Intended to reform the church, humanists taught a simplified faith accessible to any Christian who could pray directly to God for themselves.

The cult of chivalry evolved in the early 13th century and lasted into the 15th century becoming a true cultural force that influenced art, literature, and philosophy.

In 1439, a reunion agreement between the Eastern and Western churches was made. However, there was popular resistance in the East, so it wasn't until 1452 that the decree of union was officially published in Constantinople. It was overthrown the very next year by the Fall of Constantinople to the Ottoman Turks in 1453.

Compulsory resettlement returned many Greek Orthodox to Constantinople. While Islamic law did not recognize the Patriarch as a "juristic person", nor acknowledge the Orthodox Church as an institution, it did identify the Orthodox Church with the Greek community, and concern for stability allowed it to exist. The monastery at Mt. Athos prospered from the fourteenth to the sixteenth centuries. Ottomans were largely tolerant, and wealthy Byzantines who entered monastic life there were allowed to keep some control over their property until 1568.

Leaders of the church were recognized by the Islamic state as administrative agents charged with supervising its Christian subjects and collecting their taxes. Compulsory taxes, higher and higher payments to the sultan in hopes of receiving his appointment to the Patriarchate, and other financial gifts, corrupted the process and impoverished Christians. Conversion became an attractive solution.

Between 1478 and 1542, the modern Roman, Spanish, and Portuguese inquisitions were created with a much broader reach than previous inquisitions.

Until its end in 1834, the Spanish Inquisition was used to consolidate state interests. Authorized by Pope Sixtus IV in 1478, it was begun in answer to Ferdinand and Isabella's stated fears that Jewish converts were conspiring with Muslims to sabotage the new state. At the start, inquisition was so severe that the Pope attempted to shut it down. King Ferdinand is said to have threatened the Pope to prevent that. Five years after its inception, in October 1483, a papal bull conceded control of the Spanish Inquisition to the Spanish crown making it the first national, unified, centralized institution of the nascent Spanish state.

The Portuguese Inquisition was controlled by a state-level board of directors sponsored by the king, who, during this period, was generally more concerned with ethnicity than religion. According to Giuseppe Marcocci, there is a connection between the Portuguese Inquisition's growth and blood purity statutes. Anti-Judaism became part of the Inquisition in Portugal before the end of the fifteenth-century and forced conversion led many Jewish converts to Portuguese colonies in India, where they suffered as targets of the Goa Inquisition.

The Roman Inquisition operated to serve the papacy's long-standing political aims in Italy. The Roman Inquisition was bureaucratic, intellectual, and academic. It is probably best known for its condemnation of Galileo.

While the medieval Catholic church never advocated the full expulsion of Jews from Christendom, nor did the Church repudiate Augustine's doctrine of Jewish witness, canon law supported discrimination as defining Jews as heretics became increasingly common during the 15th century. Local rulers repeatedly evicted Jews from their lands and confiscated Jewish property. In 1283, the Archbishop of Canterbury spearheaded a petition demanding restitution of usury and urging the Jewish expulsion in 1290.

Frankfurt's Jews flourished between 1453 and 1613 despite harsh discrimination. They were restricted to one street and were subject to strict rules if they wished to leave this territory, but within their community, they were allowed to maintain some self-governance. They had their own laws, leaders, and a well-known Rabbinical school that also functioned as a religious and cultural centre.

In the medical, theological, and legal views of the day, it was commonly held that women were incapable of the thought needed to make moral judgments and were too weak to exercise authority. Hierarchy between the genders was seen as necessary for society. Women had no access to education within the institutions associated with the church (i.e., cathedral schools and most universities).

Where clergy was concerned, the boundary between men and women was absolute, however, there were women who became distinguished leaders of nunneries, exercising the same powers and privileges as their male counterparts. For example, Hildegard of Bingen (d. 1179), Elisabeth of Schönau (d. 1164/65), and Marie d'Oignies (d. 1213) had full autonomy over the spiritual and temporal aspects of their houses.

Among those considered to be heretics, roles for men and women varied, but the church often used the participation of women as an additional factor in demonizing a heretical movement.

Powerful and pervasive ecclesiastical reform developed from medieval critiques of the church, but the institutional unity of the church shattered. Though there was no actual schism until 1521, the Protestant Reformation (1517–1648) has been described (since the nineteenth-century) as beginning when Martin Luther, a Catholic monk advocating church reform, nailed his Ninety-five Theses to the church door in Wittenberg in 1517.

Luther's theses challenged the church's selling of indulgences, the authority of the Pope, and various teachings of the late medieval Catholic church. This act of defiance and its social, moral, and theological criticisms brought Western Christianity to a new understanding of salvation, tradition, the individual, and personal experience in relationship with God. Edicts handed down by the Diet of Worms condemned Luther and officially banned citizens of the Holy Roman Empire from defending or propagating his ideas.

The three primary traditions to emerge directly from the Reformation were the Lutheran, Reformed, and the Anglican traditions. At the same time, a collection of loosely related groups that included Anabaptists, Spiritualists, and Evangelical Rationalists, began the Radical Reformation in Germany and Switzerland. Beginning in 1519, Huldrych Zwingli spread these teachings in Switzerland leading to the Swiss Reformation.

The Roman Catholic Church responded to the Protestant challenge in what is called the Counter-Reformation or Catholic Reformation, spearheaded by a series of 10 reforming popes from 1534 to 1605, beginning with Pope Paul III (1534–1549). The Council of Trent (1545–1563) denied each Protestant claim, and laid the foundation of Roman Catholic policies up to the twenty-first-century. A list of books detrimental to faith or morals was established, the Index Librorum Prohibitorum, which included the writings of Protestants and those condemned as obscene.

New monastic orders were formed within the church, including the Society of Jesus – also known as the "Jesuits" – who adopted military discipline and a vow of loyalty to the Pope, leading them to be called "the shock troops of the papacy". They soon became the Church's chief weapon against Protestantism. Monastic reform also led to developments within orthodox spirituality, such as that of the Spanish mystics and the French school of spirituality. The Counter-Reformation also created the Uniate church which used Eastern liturgy but recognized Rome.

Quarreling royal houses, already involved in dynastic disagreements, became polarized into the two religious camps. Warfare initially broke out in the Holy Roman Empire with the minor Knights' War in 1522, then intensified in the First Schmalkaldic War (1546–1547) and the Second Schmalkaldic War (1552–1555). In 1562, France became the centre of religious warfare. The largest and most disastrous of these wars was the Thirty Years' War (1618–1648).

Theorists such as John Kelsay and James Turner Johnson argue that these wars were varieties of the just war tradition for liberty and freedom. William T. Cavanaugh identifies a view shared by many historians that the wars were not primarily religious, but were more about state-building, nationalism, and economics. Historian Barbara Diefendorf argues that religious motives were always mixed with other motives, but the simple fact of Catholics fighting Catholics and Protestants fighting Protestants is not sufficient to prove the absence of religious motives. According to Marxist theorist Henry Heller, there was "a rising tide of commoner hostility to noble oppression and growing perception of collusion between Protestant and Catholic nobles".

Until the 1300s, the official position of the Roman Catholic Church was that witches did not exist. While historians have been unable to pinpoint a single cause of what became known as the "witch frenzy", scholars have noted that, without changing church doctrine, a new but common stream of thought developed at every level of society that witches were both real and malevolent. Records show the belief in magic had remained so widespread among the rural people, that it has convinced some historians that Christianization had not been as successful as previously supposed. The main pressure to prosecute witches came from the common people, and trials were mostly civil trials. There is broad agreement that approximately 100,000 people were prosecuted, of which 80% were women, and that 40,000 to 50,000 people were executed between 1561 and 1670.

The conquest of 1453 effectively destroyed the Eastern Orthodox Church as an institution of the Christian empire as inaugurated by Constantine, sealing off Greek-speaking Orthodoxy from the West for almost a century and a half. The church was without one of its leaders, the Emperor, though it retained a patriarch in a lesser and more limited capacity. The Seljuq sultans and the Ottoman sultans were relatively tolerant, and this allowed the spiritual and cultural influence of the Eastern church, Constantinople, and Mount Athos to continue in slightly altered form among Orthodox nations. By the time of Süleyman the Magnificent (1520–1566), the patriarchate had become a part of the Ottoman system, and continued to influence the Orthodox world.

Jeremias II (1536–1595) dominated Eastern Christianity in the second half of the sixteenth century keeping Constantinople conservative and suspicious of Rome. Jeremias established contact with the new Protestant Lutherans, but nothing much resulted beyond Western Europeans becoming a little more aware of the problems of the church in captivity. Jeremias was the first Eastern patriarch to visit north-eastern Europe. Ending his visit in Moscow, he founded the Orthodox Patriarchate of Russia.

A generation after Constantinople fell to the Turks Ivan III of Muscovy adopted the style of the ancient Byzantine imperial court. This gained Ivan support among the late fifteenth and early sixteenth-century Rus' elite who saw themselves as the New Israel and Moscow as the new Jerusalem. The Church reform of Peter I in the early eighteenth-century placed the Orthodox authorities under the control of the tsar. An ober-procurator appointed by the tsar ran the committee that governed the Church after 1721 until 1918: the Most Holy Synod. The Church became involved in the various campaigns of russification and contributed to antisemitism.

The era of absolutist states followed the breakdown of Christian universalism. Abuses from political absolutism practised by kings supported by Catholicism gave rise to a virulent anti-clerical, anti-Catholic, and anti-Christian sentiment that emerged in the 1680s. Critique of Christianity began among the more extreme Protestant reformers enraged by fear, tyranny, and persecution. Secularisation spread at every level of European society.

Beginning in the 1400s, Protestants steadfastly sought religious toleration for everyone, including heretics, blasphemers, Catholics, non-Christians, and atheists. Over the next centuries, Anglicans and other Christian moderates also wrote and argued for toleration. In the 1690s, many secular thinkers rethought, on a political level, all of the State's reasons for persecution. They also began advocating for religious toleration. Concepts of freedom of religion, freedom of speech, and freedom of thought gradually became established.

In the early seventeenth-century, Baroque art, characterized by grandeur and opulence, offered the Catholic Church and secular rulers a means of expressing their magnificence and political power. This was a period of turmoil, discovery, and change, and Baroque art reflected the search for stability and order. It originated in Rome and became an international style. The church of St. Peter in Rome, St. Paul's cathedral in London, and the gardens at Versailles are probably the age's premiere examples.

Colonialism, driven by economics and politics, also opened the door for Christian missions in many new regions. According to Sheridan Gilley "Catholic Christianity became a global religion through the Spanish and Portuguese colonial empires in the sixteenth-century and French missionaries in the seventeenth and eighteenth."

However, Christian missionaries and colonial empires had separate agendas that were often in direct opposition to each other. Most missionaries avoided politics, yet over the last 500 years of missions, many vocal missionaries challenged colonial oppression, defended human rights, and opposed their own governments in matters of social justice. On the other hand, there are an equal number of examples of missionaries cooperating with colonial governments.

The sixteenth-century success of Christianity in Japan was followed by one of the greatest persecutions in Christian history. Sixteenth-century missions to China were undertaken primarily by the Jesuits. Sheridan Gilley writes that "The cruel martyrdom of Catholics in China, Indochina, Japan and Korea, another heroic missionary country, was connected to local fears of European invasion and conquest, which in some cases were not unjustified."

Historians often refer to the period from 1760 to 1830 as a "historical watershed" because it embraces the beginnings of the Industrial Revolution, the American Revolution, and the French Revolution.

In many cases, throughout this period, Christianity was weakened by social and political change. By the Nineteenth and Twentieth centuries, the influence of anticlerical socialism and communism produced secession and disruption in many locations.

After the Scientific Revolution (1600–1750), an upsurge in skepticism subjected all aspects of Western culture, including religious belief, to systematic doubt. Biblical criticism emerged (c. 1650 – c. 1800), pioneered by Protestants, using historicism and human reason to make the study of the Bible more scholarly, secular, and democratic. Depending upon how radical the individual scholar was, this produced different and often conflicting views, but it posed particular problems for the literal Bible interpretation which had emerged in the 1820s.

Before the Enlightenment of the eighteenth-century, liberalism was synonymous with Christian Idealism in that it imagined a liberal State that embraced political and cultural tolerance and freedom. Later, liberalism embraced seventeenth-century rationalism, which was attempting to "wean" Christianity from its "irrational cultic" roots. This liberalism lost touch with the necessity of faith and ritual in maintaining Christianity, which led to liberalism's decline and the birth of fundamentalism.

Fundamentalist Christianity arose in the late nineteenth and early twentieth-century as a reaction against modern rationalism. The Roman Catholic Church became increasingly centralized, conservative, and focused on loyalty to the Pope. Early in the twentieth-century, the Pope required Catholic Bible scholars who used biblical criticism to take an anti-modernist oath.

In the same period (1925), supporters of a relatively new, loosely organized, and undisciplined Protestant fundamentalism participated in the Scopes trial. By 1930, the movement appeared to be dying. Later in the 1930s, Neo-orthodoxy, a theology against liberalism with a reevaluation of Reformation teachings, began uniting moderates of both sides. In the 1940s, "new-evangelicalism" established itself as separate from fundamentalism.

Religious revival, known as the First Great Awakening, swept through the American colonies between the 1730s and the 1770s. Verbal battles over the movement raged at both the congregational and denominational levels creating divisions which became 'Parties', which turned political and eventually led to critical support for the American Revolution.

In places like Connecticut and Massachusetts, where one denomination received state funding, churches now began to lobby local legislatures to end that inequity. In 1791, the United States became the first predominantly Christian nation to mandate the separation of church and state. Theological pluralism became the new norm.

Scholars have identified a positive correlation between the rise of Protestantism and human capital formation, the Protestant work ethic, economic development, and the development of the state system. Max Weber says Protestantism contributed to the development of banking across Northern Europe and gave birth to Capitalism. However, the urbanization and industrialization that went hand in hand with capitalism created a plethora of new social problems. In Europe and North America, both Protestants and Catholics provided massive aid to the poor, supporting family welfare, medicine, and education.

France also experienced revolution, and by 1794, radical revolutionaries attempted to violently 'de-Christianize' France in what some scholars have termed a "deliberate genocidal policy of extermination" of Catholics in the Vendée region. When Napoleon came to power, he acknowledged Catholicism as the majority view and tried to make it dependent upon the state. For Eastern Orthodox church leaders, the French Revolution meant Enlightenment ideas were too dangerous to embrace.

The Second Great Awakening (1800–1830s) extolled moral reform as the Christian alternative to armed revolution. These reformers established nationwide societies, separate from any individual church, to begin social movements concerning abolition, women's rights, temperance and literacy. Developing nationwide organizations was pioneering, and many businesses adopted the practice leading to the consolidations and mergers that reshaped the American economy of the nineteenth-century. The second awakening produced the Latter Day Saint movement, the Restoration Movement and the Holiness movement.

The Third Great Awakening began in 1857 and was most notable for taking the movement throughout the world, especially in English-speaking countries. Restorationists were prevalent in America. They have not described themselves as a reform movement but have, instead, described themselves as restoring the Church to its original form as found in the book of Acts. Restorationism gave rise to the Stone-Campbell Restoration Movement, Adventism, and the Jehovah's Witnesses.

For over 300 years, many Christians in Europe and North America participated in the Trans-Atlantic slave trade, which had begun in the sixteenth-century. Moral objections had arisen immediately but had little impact. By the eighteenth-century, individuals from the Religious Society of Friends (Quakers) were followed by certain Methodists, Presbyterians and Baptists, in writing and disseminating pamphlets against both the trade and slavery itself. In the years after the American Revolution, black congregations led by black preachers provided an institutional base for keeping abolitionism alive. In the early nineteenth-century, some American Protestants organized the first anti-slavery societies. According to historian David Eltis, the ideology of abolitionism eventually ended the Trans-Atlantic slave trade, changing the economic and human history on three continents.

Protestant missionaries had a significant role in shaping multiple nations, cultures, and societies. Women made major contributions. A missionary's first job was to get to know the indigenous people and work with them to translate the Bible into their local language. Approximately 90% were completed. Often, the process also generated a written grammar, a lexicon of native traditions, and a dictionary of the local language. These were used to teach in missionary schools, resulting in the spread of literacy. According to historian Lamin Sanneh, Protestant missionaries began the "largest, most diverse and most vigorous movement of cultural renewal in history" in Africa. Many native cultures responded similarly.

The Bolsheviks and other Russian revolutionaries saw the Church, like the tsarist state, as an enemy of the people. Criticism of atheism was strictly forbidden and sometimes led to imprisonment. Some actions against Orthodox priests and believers included torture, being sent to prison camps, labour camps or mental hospitals, and execution.

Historian Scott Kenworthy describes the persecution of the Russian Orthodox Church under communism as "unparalleled by any in Christian history". In the first five years after the October Revolution, one journalist reported that 28 bishops and 1,200 priests were executed. Others report that 8,000 people were killed in 1922. The League of Militant Atheists adopted a five-year plan in 1932 "aimed at the total eradication of religion by 1937".

Despite oppression and martyrdom under hostile rule, the Orthodox churches of the twentieth-century continued to contribute to theology, spirituality, liturgy, music, and art. Kenworthy adds that "Important movements within the church have been the revival of a Eucharistic ecclesiology, of traditional iconography, of monastic life and spiritual traditions such as Hesychasm, and the rediscovery of the Greek Church Fathers".

In the early twentieth-century, European states were advocating the separation of church and state, while also establishing authoritarian governments and state-supported churches. Such consanguinity would, after 1945, implicate the church in abuses of power.

Pope Pius XI declared in Mit brennender Sorge (English: "With rising anxiety") that fascist governments had hidden "pagan intentions" and expressed the irreconcilability of the Catholic position with totalitarian fascist state worship which placed the nation above God, fundamental human rights, and dignity.

In Poland, Catholic priests were arrested and Polish priests and nuns were executed en masse.

Most leaders and members of the largest Protestant church in Germany, the German Evangelical Church, which had a long tradition of nationalism and support of the state, supported the Nazis when they came to power. A smaller contingent, about a third of German Protestants, formed the Confessing Church which opposed Nazism.

Nazis interfered in The Confessing Church's affairs, harassed its members, executed mass arrests, and targeted well-known pastors like Martin Niemöller and Dietrich Bonhoeffer. Bonhoeffer, a pacifist, was arrested, found guilty in the conspiracy to assassinate Adolf Hitler, and executed.

After World War II, Christianity became a global religion, faced major challenges, broke down denominational boundaries, was impacted by war, and gave substantive aid to the oppressed.

The world's largest religion has been Christianity since the eighteenth-century. Before 1945, about a third of the people in the world were Christians (with about half of those Roman Catholic), and about 80% of all Christians lived in Europe, Russia, and the Americas. After the collapse of communism in Eastern Europe and Russia, Christianity grew and expanded there. At the same time, it spread in Africa and Asia. By 2000, the percentage of Christians in the West dropped to around 40 percent, while the proportion living in Asia and Africa rose to 32 percent. Christianity's population center shifted east and south, making it a truly global religion.

In the first quarter of the twenty-first century, most Christians live outside North America and Western Europe. White Christians are a global minority, and slightly over half of worldwide Christians are female. It remains the world's largest religion into the twenty-first-century with roughly 2.4 billion followers, constituting around 31.2% of the world population.

In 1900, under colonial rule, there were just under 9 million Christians in Africa. By 1960, and the end of colonialism, there were about 60 million. By 2005, African Christians had increased to 393 million, about half of the continent's total population. Population in Africa has continued to grow with the percentage of Christians remaining at about half in 2022. This expansion has been labeled a "fourth great age of Christian expansion".

Christianity has grown rapidly in China and the rest of Southeast Asia, especially Korea, where it grew faster after colonialism than before it. A rapid expansion of charismatic Christianity began in the 1980s, leading Asia to rival Latin America in the population of Charismatic and Pentecostal Christians. The Council on Foreign Relations data shows a 10% yearly growth in Chinese Christian populations since 1979. Increasingly, this includes young people more than any other group.

Traditional Christianity has faced multiple challenges in the twentieth-century. In the U.S., Pew has reported that "As recently as the early 1990s, about 90% of U.S. adults identified as Christians. But [in 2015], about two-thirds of adults are Christians". Secularism, the changing moral climate in the West, and various types of political opposition have led to a decline in church attendance. Hugh McLeod writes that,

The most powerful and effective criticism of Christianity in the twentieth-century has been the charge that it has been too closely identified with the rich and powerful, and too ready to legitimate the status quo. These political criticisms have had a far wider impact than those deriving from scientific or philosophical objections to religion.

Highly authoritarian and totalitarian governments have brought about crises and decline in churches in many areas. From 1945 into the 1980s, the world's first Marxist super-power, along with the many other communist governments, pursued anti-religious policies that were often violent. In 2013, 17 Muslim majority states reported 28 of the 29 types of religious discrimination against 45 of the 47 religious minorities in their countries, including Christianity. Anti-Christian persecution has become a consistent human rights concern.

The challenges of secularism, and the changing moral climate of the 1960s and 1970s, caused controversy within the churches concerning sexual ethics, gender, and exclusivity. A growing demand for greater individual freedom led to new forms of religion that embrace the sacred as a deeper understanding of the self. This "New Age" spirituality is private and individualistic and differs radically from Christian tradition, dogma, and ritual.

Collaboration between Protestants and Catholics made little progress until 11 October 1962, when Pope John XXIII opened the Second Vatican Council, the 21st ecumenical council of the Catholic Church. On 21 November 1964, the Second Vatican Council published Unitatis Redintegratio, stating that Roman Catholic ecumenical goals are to establish full communion amongst all the various Christian churches including Protestants. Amongst Evangelicals, there is no agreed-upon definition, strategy, or goal for ecumenism. Different theologies on the nature of the Church have produced some hostility toward it instead.

While the sentiment is widespread that ecumenism at the upper levels of leadership has stalled, the trend at the local level has been toward discussion and prayer meetings, pulpit exchanges, and shared social action. The common threat of secularisation and a recognition of the destructive potential of religious hatred has encouraged cooperation between churches. In the U.S. there has been an increase in inter-marriage. Almost 40% of couples married since 2010, compared to 19% before 1960, have married someone outside of their faith, according to Pew Research Center.

Christianity is still diverse, and Christians still disagree, but the grounds have changed to topics that engage the deepest and most controversial issues of the twenty-first-century – "race, gender, colonialism, and liberation" – bringing these to the forefront of the larger more traditional Christian agenda. In Hugh MacLeod's view, "A liberal Catholic is likely to have a lot in common with a liberal Methodist", and this commonality is only likely to increase with the influence of the internet.

The Prosperity gospel formed as an adaptation of Pentecostalism. It challenges traditional Christianity because it has moved away from the Reformation view of biblical authority to the authority of personal charisma. Begun in the twentieth-century's last decades, it has become a trans-national movement. In 2000, approximately one-quarter of all Christians worldwide were part of Pentecostalism and its associated movements. By 2025, Pentecostals are expected to constitute one-third of the nearly three billion Christians worldwide making it the largest branch of Protestantism and the fastest-growing religious movement in global Christianity.

The multiple wars of the twentieth-century brought questions of theodicy to the forefront. Wars have had contradictory effects on the church, sometimes producing a loss of faith in human solutions, an upsurge in religiosity and patriotism, or an alienation from Christianity. For the first time since the pre-Constantinian era, Christian pacifism became an advocated Christian option to war in the twentieth-century.

The nineteenth-century revolutions that established Orthodoxy in the Serbian, Greek, Romanian, and Bulgarian nations were changed in the twentieth-century from a universal church into a series of national churches that became subordinate to nationalism and the state.

Liberation theology has been especially effective in aiding the Latin American poor. To redeem the institutions of society using the "kingdom ideals" from Jesus' Sermon on the Mount, the Social Gospel combined with liberation theology to redefine social justice, focus on the community's sins, and expose institutionalized sin.

Originating in America in 1966, Black theology developed a combined social gospel and liberation theology that mixes Christianity with questions of civil rights, aspects of the Black Power movement, and responses to black Muslims claiming Christianity was a "White man's" religion. Spreading to the United Kingdom, then parts of Africa, confronting apartheid in South Africa, Black theology explains Christianity as liberation for this life not just the next.

The feminist movement of the mid to late twentieth-century began with an anti-Christian ethos but soon developed a significant and influential Feminist theology dedicated to transforming the churches and society. In the last years of the twentieth-century, the re-examination of old religious texts through diversity, otherness, and difference developed womanist theology of African-American women, the "mujerista" theology of Hispanic women, and insights from Asian feminist theology.

After World War II, Christian missionaries played a transformative role in many colonial societies, moving them toward independence through decolonization. In the mid to late 1990s, postcolonial theology emerged globally from multiple sources. It analyzes structures of power and ideology to recover what colonialism erased or suppressed in indigenous cultures.

The missionary movement of the twenty-first-century has transformed into a multi-cultural, multi-faceted global network of NGO's, short-term amateur volunteers, and traditional long-term bilingual, bicultural professionals who focus on evangelism and local development and not on 'civilizing' native people.

• Chronology of Jesus
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