World Religions

The Birth of Christianity

Day 2Abrahamic tradition~2.3–2.6 billion adherents

Christianity is the largest religion in the world today — roughly one in three people is a Christian. Yet its beginning was utterly obscure: a Jewish teacher executed under Roman rule in Palestine, and a small band of followers who believed he had risen from the dead. How did a messianic movement inside Judaism grow, within three centuries, into a universal faith spanning the Roman Empire and eventually the globe? That is the thread of this issue.

"Love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength… Love your neighbor as yourself."— Jesus quoting Deuteronomy and Leviticus as the "greatest commandment," Mark 12:29–31

Origins & History

From a man of Nazareth to an empire's faith

The source of Christianity is the historical Jesus (c. 4 BCE – c. 30 CE): an Aramaic-speaking Galilean Jew living in the Second Temple period. He preached that "the kingdom of God is near," gathered disciples, and was crucified in Jerusalem on political charges under the Roman governor Pontius Pilate. For believers, the decisive event was what followed — the Resurrection: the disciples proclaimed that Jesus had risen and was the promised Messiah (in Greek, Christos, "Christ" meaning "the anointed one"). Christianity as a movement of faith was born in that proclamation.

At first it was only a sect within Judaism. What turned it universal was Paul of Tarsus (Paul, once a persecutor, then a convert): he insisted that Gentiles need not first be circumcised or keep the Law of Moses to join, and carried the message into the Greco-Roman world. Paul's letters (c. 50s CE) are the earliest written parts of the New Testament, predating the four Gospels. By the end of the 1st century, Christianity was gradually parting ways with Judaism to become an independent tradition.

For nearly three centuries after, Christians were by turns tolerated and persecuted across the Roman Empire, yet kept growing. The turning point came in the 4th century: the Edict of Milan (313), issued by Constantine and Licinius, granted Christianity legal standing and ended systematic persecution. The Council of Nicaea (325) — convened by Constantine with some three hundred bishops — addressed the Arian dispute over "whether Christ is of one substance with God," affirming that Jesus is "of the same substance" (homoousios) as the Father and drafting the core of the Nicene Creed. In 380, Theodosius I issued the Edict of Thessalonica, formally making Nicene Christianity the state religion of the Roman Empire. The faith of a crucified man had become the faith of the empire.

The spine of the first three centuries: from a proclamation about "resurrection" to the empire's official faith. The great schisms (1054, 1517) are left for Day 3–5.

Distribution Today

The world's largest religion, its center shifting south

Christians number roughly 2.3–2.6 billion worldwide (Pew's 2020 estimate is about 2.3 billion, ~29% of the world; other bodies estimate near 2.6 billion, ~31%, for 2025) — by any measure the largest religion on earth, about a third of humanity. Institutionally it falls into three broad branches:

Catholicism: about 1.3 billion, the largest single institution, centered on the Bishop of Rome (the Pope) — Day 3.
Protestantism: about 900 million–1 billion, the sum of the many denominations that broke off after the Reformation — Day 5.
Eastern Orthodoxy: about 200–300 million, the Eastern church tradition, with no single head — Day 4.

The key trend is a shift southward: Christianity was born in the Middle East and peaked in Europe, but today roughly 70% of Christians live in the "Global South" — sub-Saharan Africa, Latin America, Asia. Africa alone has more than 750 million Christians and is growing fast, while religious practice in Europe contracts. Unlike Judaism, Christianity is a classic universalizing, missionary religion — the Great Commission to "make disciples of all nations" has been the inner engine of two millennia of expansion.

Core Beliefs

Incarnation, and salvation through faith

The Trinity

Christianity inherits Judaism's strict monotheism, yet advances a distinctive and puzzling formulation: the one God exists as three "persons" — Father, Son, and Holy Spirit — one being, one substance. This is not three gods but three persons of a single God — precisely what the Council of Nicaea sought to clarify, and the most fundamental point of divergence from Judaism and Islam (both of which see it as a departure from monotheism).

Incarnation and salvation

The central belief is that Jesus is both fully God and fully human (the Incarnation). Christianity holds that humanity is estranged from God by "original sin" and cannot bridge the gap on its own; Jesus's death and resurrection bear the sins of the world and open a path to salvation. That path lies in faith (especially for Protestants) and the grace of God rather than in keeping the Law alone — the key structural turn away from its parent, Judaism.

The Bible

Its scripture is the Bible: the Old Testament inherited from Jewish tradition, and the New Testament recording Jesus's life, the early church, and the apostolic letters. For Christians the whole Bible points along one line — God's redemption of humanity.

Principles & Practice

A life centered on one who rose

At the heart of Christian practice is a relationship with the Christ believed to have risen, and living out the commandment to "love God and neighbor." Elements common across denominations include:

Baptism: marked with water, symbolizing the washing away of sin and entry into the community of faith — for most traditions the starting point of membership.
The Eucharist (Communion / Lord's Supper): bread and wine commemorating Jesus's Last Supper and death; denominations understand its meaning differently (Real Presence vs. symbolic remembrance), a significant internal divide.
Sunday worship and prayer: gathering on Sunday (in memory of the Resurrection) to worship, read scripture, and hear preaching; prayer is the daily way of speaking with God.
Festivals: Christmas (Jesus's birth), Easter (the Resurrection, the most central feast), along with Advent, Lent, Pentecost, and others, cyclically re-enacting the story of salvation.
Ethics and mission: an ethic centered on "love your neighbor" and care for the weak; the Great Commission (to spread the gospel) drove historic missions, schooling, and charity — intertwined, too, with complex legacies such as colonialism.

One caveat: internal differences are vast. Catholicism, Orthodoxy, and Protestantism diverge in their sources of authority, understanding of the sacraments, elaborateness of liturgy, and church structure — this issue sketches only their shared skeleton; the distinct face of each branch is left for Day 3–5.

Common Misconceptions

Misconception: Christianity was a unified religion from the start.
At first it was a messianic movement within Judaism; its early doctrines (especially Christ's divinity) took shape only through centuries of debate and several ecumenical councils, and it was never truly monolithic.

Misconception: the Trinity means "believing in three gods."
Christianity regards itself as still strictly monotheistic: one God, three persons, one substance. The formulation exists precisely to hold faith in Jesus while affirming one God.

Misconception: Jesus founded the religion called "Christianity."
The historical Jesus was a Jew his whole life, preaching within a Jewish context. Christianity as a separate religion was built up after him by the disciples, Paul, and the later church.

Misconception: Christianity is a "Western (white) religion."
It originated in the Middle East, and today roughly 70% of believers are in the Global South; Africa, Latin America, and Asia are where it grows fastest now.

Cross-Tradition Bridge · Contemporary Standing

With Judaism: one root, then a fork. Christianity carries forward Judaism's one God, scripture (the Old Testament), messianic hope, and ethics, yet parts ways over "has the Messiah come?" — it proclaims Jesus as that Christ, and further advances the Incarnation and the Trinity, which Judaism does not accept (see Day 1).

With Islam: mutual recognition, real difference. Islam honors Jesus (Isa) as a great prophet, affirms his virgin birth, yet denies his divinity and his crucifixion, and views the Trinity as a departure from monotheism. The three Abrahamic faiths share a lineage of prophets and scriptures; their disagreement concentrates most sharply on the identity of Jesus (see Day 6).

Contemporary standing. Christianity is undergoing a deep geographic and morphological shift: secularization and membership decline in Europe and North America, while the Global South booms, Pentecostal and charismatic movements especially. How it understands itself in a culturally plural world with varied church-state arrangements is an ongoing question for the largest religion.

For Reflection

Why did a movement whose founder was executed as a criminal not collapse, but instead grow?

In the ancient Mediterranean, crucifixion was a supremely shameful execution, one that should have ended any movement centered on the dead man. Christianity instead retold this "failure" as the core of salvation — the cross not an end but a turning point. The key driving force was the disciples' conviction of the "resurrection," and the community it produced, willing to suffer and even die for the faith. Historically, growth was also aided by its universal openness (across ethnic lines), a practical ethic of caring for the sick, poor, and weak, and the convenience of the empire's transport networks. Why a narrative of "suffering and resurrection" could hold such cohesive power is a classic case for understanding religious persuasion.

How did placing "faith" above "keeping the Law" reshape the religion's form?

Judaism makes observing the commandments (the Law) the core of responding to God; Paul stressed that a person is "justified by faith" and saved by grace. This turn removed the thresholds of circumcision, dietary law, and the rest for Gentiles, letting Christianity swiftly cross ethnic boundaries toward universality — the structural reason its reach differed so sharply from Judaism's. But it also brought new tension: how to place faith in relation to works, and whether grace might slight ethical practice. This question was re-argued repeatedly in later eras (especially the Reformation, Day 5), and remains a deep fault line between denominations.

When a persecuted faith becomes an empire's state religion, what does it gain, and what might it lose?

From persecution to becoming the state religion in 380, Christianity gained unprecedented resources, institutions, and reach, and its doctrine was unified through the ecumenical councils. But historians often note the cost: once faith is joined to political power, the church can be drawn into the empire's logic, "conversion" sometimes mixes with social pressure rather than pure choice, and dissenters are more readily branded heretics. This "Constantinian turn" is a recurring motif in religious history — the dilemma any faith faces when it acquires worldly power. There is no standard answer, but it is worth every tradition's self-examination.

This piece introduces the origins and shared skeleton of Christianity from a neutral, comparative standpoint, aiming to present its internal logic and diversity without praise or ranking. Population figures are recent estimates and are given as ranges because definitions and methods differ; dates (such as the crucifixion) follow the scholarly mainstream and remain academically debated.