Christianity is a monotheistic Abrahamic religion based on the life and teachings of Jesus of Nazareth. Its adherents, known as Christians, believe Jesus is the Son of God and savior of humanity, whose coming as the Messiah (Christ) was prophesied in the Old Testament of the Bible, and chronicled in the New Testament.
Christianity began as a Second Temple Judaic sect in the 1st century in the Roman province of Judea. Jesus' apostles and their successors, the Apostolic Fathers, spread it across large parts of the Middle East, Europe, Ethiopia, Transcaucasia and some other parts of Asia, despite initial persecution. Emperor Constantine the Great converted to Christianity and decriminalized it in the Edict of Milan (313). He convened the First Council of Nicaea (325), where Early Christianity was consolidated into what would become the state religion of the Roman Empire (380). The council formulated the Nicene Creed (325), and the Church Fathers supervised the compilation of the Christian Bible (5th century). The period of the first seven ecumenical councils is sometimes referred to as the Great Church, the united full communion of the Catholic Church, Eastern Orthodox Church, and Oriental Orthodoxy before their schisms. Oriental Orthodoxy split after the Council of Chalcedon (451) over differences in Christology. Eastern Orthodox Church and the Catholic Church separated in the East–West Schism (1054), especially over the authority of the Pope. Similarly, Protestantism split in numerous denominations from the Catholic Church in the Protestant Reformation (16th century) over theological and ecclesiological disputes.
Christianity and Christian ethics have played a prominent role in the development of Western civilization, particularly around Europe during Late Antiquity and the Middle Ages. Following the Age of Discovery (15th–17th century), Christianity was spread into the Americas, Oceania, Sub-Saharan Africa and the rest of the world via missionary work and colonization.
It is the world's most populous religious group, with over 2.4 billion followers, or 33% of the global population, comprising a majority of the population in about two-thirds of the countries in the world. Today, the four largest branches of Christianity are the Catholic Church (1.3 billion), Protestantism (920 million), the Eastern Orthodox Church (260 million) and Oriental Orthodoxy (86 million).