What is Christianity?

Why Christianity?

Because Christianity is one of the most influential religious movements in human history, yet many people—including Christians themselves—do not fully understand its origins, development, or the many branches it has produced over the past two thousand years. For many of us, Christianity was inherited long before it was examined. It came to us through family, school, church, rituals, holidays, sacraments, and community expectations. In my own case, Catholicism—a tradition that claims direct continuity with the early apostolic church—was woven into the structure of daily life: K–12 Catholic education, Sunday Mass with my family, and the sacraments of childhood, including First Confession, First Holy Communion, and Confirmation. Over time, however, an inherited religious identity can become complicated. Some people remain deeply committed to organized religion. Others drift away. Some reject it entirely. My own Catholic identity and belief system have changed significantly from my twenties to my sixties, but the early memories of Catholic worship, doctrine, and spiritual formation remain deeply imprinted.

Christianity also matters because it continues to shape public life in ways that now reach far beyond private belief or Sunday worship. In the United States, Christian language and identity are invoked in debates over law, education, sexuality, abortion, nationalism, immigration, and the role of government itself. In recent years, that influence has become especially visible through the rise of Christian nationalism—the belief that the United States should be defined, governed, or morally ordered as an explicitly Christian nation. PRRI’s 2025 American Values Atlas found that roughly one-third of Americans qualify as Christian nationalism “Adherents” or “Sympathizers,” showing that this is no longer a fringe idea but a measurable force in public opinion. Recent examples include state efforts to place the Ten Commandments in public-school classrooms, such as Louisiana’s 2024 law (later struck down by the Fifth Circuit in 2025) and Texas’s 2025 law requiring displays in classrooms; Oklahoma’s attempts to mandate Bible instruction and purchase Bibles for public schools; and federal actions such as the creation of a White House Faith Office, a Religious Liberty Commission, and a task force aimed at “eradicating anti-Christian bias.” These developments do not represent all Christians, many of whom strongly support church-state separation, but they show how Christianity is increasingly used as a political identity, legal argument, and cultural organizing force. Before Christianity becomes a slogan, a voting cue, a cultural weapon, or a badge of national belonging, it deserves to be understood on its own historical and theological terms. That requires history.

What is Christianity?

This question begins with Jesus of Nazareth. Jesus was a Jew. He lived in Roman-occupied Judea and Galilee, taught within the world of Second Temple Judaism, quoted the Hebrew Scriptures, observed Jewish festivals, and gathered Jewish disciples. Christianity did not begin as a fully formed religion separate from Judaism, but as a Jewish messianic movement centered on the belief that Jesus was the Christ. From there, the movement spread through the Roman Empire, developed doctrines about Jesus’ divinity, organized itself into churches, debated its core beliefs, and eventually divided into major traditions: Roman Catholic, Eastern Orthodox, Protestant, Evangelical, Pentecostal, and many others.

From Local Communities to the Early Catholic Church

The earliest Jesus movement began in Jerusalem among Jewish followers who proclaimed that Jesus had been crucified and raised from the dead. In Acts, the movement spreads from Jerusalem to Antioch, Asia Minor, Greece, and Rome. The term Christian appears in the New Testament in connection with Antioch, where followers of Jesus were identified as a distinct group. The first major internal crisis concerned Gentile converts. Did non-Jews who followed Jesus need to be circumcised and observe the Mosaic law? The Council of Jerusalem, traditionally dated around the middle of the first century, resolved that Gentiles could enter the movement without full conversion to Judaism. This decision did not create a separate denomination, but it opened Christianity to a much broader, multiethnic future. By the second and third centuries, Christian communities were found across the Roman Empire and beyond. They developed patterns of worship, episcopal leadership, baptismal instruction, creedal confession, and theological interpretation. The church increasingly defined itself against rival interpretations of Jesus, including Marcionite, Gnostic, Montanist, and other movements.

Constantine, Councils, Creeds, and the Definition of Orthodoxy

In the early fourth century, the Roman emperor Constantine dramatically changed Christianity’s place in the empire. After attributing his victory at the Battle of the Milvian Bridge in 312 to the Christian God, he favored the faith and, with Licinius, issued the Edict of Milan in 313, granting Christians legal toleration and restoring confiscated property. Although Constantine was baptized only shortly before his death in 337—and did not make Christianity Rome’s official religion—his patronage ended major persecution, funded churches, and gave bishops greater public influence, while also beginning a complicated relationship between church and imperial power. The century’s councils sought to settle disputes about Christian belief, especially the identity of Jesus. At the Council of Nicaea in 325, bishops rejected the teaching that the Son was a created being and affirmed that Jesus Christ is fully divine, “of one substance” with the Father. The Council of Constantinople in 381 expanded and reinforced the teaching of the earlier Council of Nicaea, producing what is commonly known as the Nicene-Constantinopolitan Creed. This creed remains one of the central statements of Christian belief and is still professed regularly in worship today, including at Roman Catholic Mass on Sundays and solemnities, as well as in Eastern Orthodox, Oriental Orthodox, Anglican, and many Protestant traditions. Its words summarize core Christian beliefs about God the Father, Jesus Christ the Son, the Holy Spirit, the Church, baptism, resurrection, and eternal life.

The East-West Schism: Roman Catholic and Eastern Orthodox

From the fourth century onward, Christianity’s growing institutional unity was accompanied by disputes over discipline, doctrine, worship, and authority. The Latin-speaking church in the West and the Greek-speaking churches in the East gradually developed different political relationships, liturgical customs, theological expressions, and models of leadership. Rome increasingly emphasized the universal authority of the pope, whereas the Eastern patriarchates favored a more conciliar structure. Disagreements over papal jurisdiction, the interpretation and application of the Nicene Creed, and centuries of political rivalry culminated symbolically in the mutual excommunications of 1054, producing the enduring division between the Roman Catholic Church and the Eastern Orthodox churches. Although those excommunications were lifted in 1965, full communion has not been restored.

The Protestant Reformation and the Formation of New Denominations

The Protestant Reformation began in the context of late medieval reform debates. Martin Luther’s Ninety-five Theses of 1517 protested abuses connected with papal indulgences and became a catalyst for a wider crisis in Western Christianity. Luther and his followers were eventually excommunicated, and Lutheran churches developed around doctrines such as justification by faith, the authority of Scripture, and reform of worship. Lutheranism arose from Martin Luther’s reforms in German-speaking Europe and Scandinavia. It retained many liturgical and sacramental elements of Western Christianity while rejecting papal supremacy and emphasizing justification by grace through faith.

The Reformed tradition developed through figures such as Huldrych Zwingli, John Calvin, and their successors in Switzerland, France, Scotland, the Netherlands, and beyond. Presbyterianism is a Reformed church tradition associated especially with Scotland and governed by councils of elders rather than bishops.

The Church of England separated from Roman authority under Henry VIII in the sixteenth century, initially for political and dynastic reasons. Under later reformers, especially Thomas Cranmer, Anglicanism incorporated Protestant theology while retaining episcopal structure and many liturgical features inherited from medieval Catholicism. The Anglican Communion later spread globally through British colonial and missionary expansion.

The Radical Reformation produced Anabaptist communities that rejected infant baptism, emphasized voluntary discipleship, and often advocated separation from state control of the church. Mennonites, Amish, Hutterites, and related groups descend from this tradition.

Post-Reformation and Modern Denominational Growth

Baptists emerged as an English-speaking denomination in the seventeenth century, closely related to Puritan and Congregational contexts. They emphasized believer’s baptism, congregational church governance, and religious liberty.

Methodism began in eighteenth-century England through John and Charles Wesley as a movement of renewal within the Church of England. It emphasized personal conversion, disciplined spiritual practice, lay preaching, concern for the poor, and holiness of life. It eventually became a separate church tradition.

The nineteenth century saw many new movements, especially in Britain and North America. The Holiness movement grew out of Methodist spirituality. Adventist groups emphasized Christ’s return and Sabbath observance. The Stone-Campbell Restoration Movement sought to restore New Testament Christianity and gave rise to Churches of Christ, Christian Churches, and Disciples of Christ. Latter-day Saints and Jehovah’s Witnesses also arose in this period, though their non-Nicene doctrines place them outside the boundaries of orthodox Christianity for Catholic, Orthodox, and most Protestant churches.

Modern Pentecostalism grew from Holiness and revivalist contexts and expanded dramatically after the Azusa Street revival in Los Angeles in 1906. Pentecostal churches emphasize the gifts of the Holy Spirit, healing, prophecy, speaking in tongues, and lively worship. The later charismatic movement spread Pentecostal-style spirituality into Catholic, Orthodox, Anglican, and mainline Protestant settings.

Evangelicalism is a broad movement, not a single denomination. It emphasizes conversion, the authority of the Bible, the centrality of Christ’s death and resurrection, and active mission. Nondenominational churches often share evangelical or charismatic theology while minimizing formal denominational labels. In the late twentieth and twenty-first centuries, these churches became especially prominent in the United States, Latin America, Africa, and parts of Asia.

Conclusion

The Christian movement began as a Jewish messianic community centered on Jesus and gradually became a global religion with many churches, communions, and denominations. Its divisions were shaped by theology, language, empire, worship, church authority, reform, revival, migration, and culture. The resulting diversity is not simply a story of fragmentation; it is also a story of adaptation, expansion, and ongoing debate over what it means to follow Jesus Christ.

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AI disclosure: ChatGPT Pro GPT-5.6-Sol assisted with research organization and editing. The author independently verified the article’s factual claims and sources and takes responsibility for the final text.