Christianity is an Abrahamic monotheistic religion based on the life and teachings of Jesus of Nazareth. Its adherents, known as Christians, believe that Jesus is the Christ, the Son of God, and the savior of all people, whose coming as the Messiah was prophesied in the Hebrew Bible, called the Old Testament in Christianity, and chronicled in the New Testament. It is the world's largest religion with about 2.4 billion followers.

Christianity began as a Second Temple Judaic sect in the 1st century in the Roman province of Judea. Jesus' apostles and their followers spread around Syria, the Levant, Europe, Anatolia, Mesopotamia, Transcaucasia, Egypt, and Ethiopia, despite initial persecution. It soon attracted gentile God-fearers, which led to a departure from Jewish customs, and, after the Fall of Jerusalem, AD 70 which ended the Temple-based Judaism, Christianity as a religion began.

Emperor Constantine the Great converted to Christianity (312) and decriminalized it in the Roman Empire by the Edict of Milan (313), later convening the Council of Nicaea (325) where Early Christianity was consolidated into what would become the State church of the Roman Empire (380). The early history of Christianity's united church before major schisms is sometimes referred to as the "Great Church". The Church of the East split after the Council of Ephesus (431) and Oriental Orthodoxy split after the Council of Chalcedon (451) over differences in Christology, while the Eastern Orthodox Church and the Catholic Church separated in the East-West Schism (1054), especially over the authority of the bishop of Rome. Similarly, Protestantism split in numerous denominations from the Latin Catholic Church in the Reformation era (16th century) over theological and ecclesiological disputes, most predominantly on the issue of justification and the primacy of the bishop of Rome. 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.

Christianity remains culturally diverse in its Western and Eastern branches, as well as in its doctrines concerning justification and the nature of salvation, ecclesiology, ordination, and Christology. The four largest branches of Christianity are the Catholic Church (1.3 billion/50.1 %), Protestantism (920 million/36.7 %), the Eastern Orthodox Church (260 million) and Oriental Orthodoxy (86 million/both together 11.9%), amid various efforts toward unity (ecumenism). Their creeds generally hold in common Jesus as the Son of God--the logos incarnated--who ministred, suffered, and died on a cross, but rose from the dead for the salvation of mankind; as referred to as the gospel, meaning the "good news", in the Bible (scripture). Describing Jesus' life and teachings are the four canonical gospels of Matthew, Mark, Luke and John with the Jewish Old Testament as the gospel's respected background.