Christianity began as a Second Temple Judaic movement in the 1st century in the Roman province of Judea, from where it spread throughout and beyond the Roman Empire.

Christianity "emerged as a movement of Judaism in Roman Judea" in the syncretistic Greco-Roman world of the 1st century AD, which was dominated by Roman law and Hellenistic culture. It started with the ministry of Jesus, who proclaimed the coming of the Kingdom of God. After his death by crucifixion, some of his followers are said to have seen Jesus, and proclaimed him to be alive and resurrected by God. The resurrection of Jesus "signalled for earliest believers that the days of eschatological fulfillment were at hand," and gave the impetus in certain Christian sects to the exaltation of Jesus to the status of divine Son and Lord of God's Kingdom and the resumption of their missionary activity.

Traditionally, the years following Jesus until the death of the last of the Twelve Apostles is called the Apostolic Age, after the missionary activities of the apostles. According to the Acts of the Apostles (the historical reliability of which is disputed), the Jerusalem church began at Pentecost with some 120 believers, in an "upper room," believed by some to be the Cenacle, where the apostles received the Holy Spirit and emerged from hiding following the death and resurrection of Jesus to preach and spread his message.

The Christian Testament writings depict what orthodox Christian churches call the Great Commission, an event where they describe the resurrected Jesus Christ instructing his disciples to spread his eschatological message of the coming of the Kingdom of God to all the nations of the world. The most famous version of the Great Commission is in Matthew 28:16–20, where on a mountain in Galilee Jesus calls on his followers to make disciples of and baptize all nations in the name of the Father, the Son, and the Holy Spirit.

Paul's conversion on the road to Damascus is first recorded in Acts 9:13–16. Peter baptized the Roman centurion Cornelius, traditionally considered the first Gentile convert to Christianity, in Acts 10. Based on this, the Antioch church was founded. It is also believed that it was there that the term Christian was coined.

After the crucifixion of Jesus, Christianity first emerged as a sect of Judaism as practiced in the Roman province of Judea. The first Christians were all Jews, who constituted a Second Temple Jewish sect with an apocalyptic eschatology.

The Jerusalem community consisted of "Hebrews," Jews speaking both Aramaic and Greek, and "Hellenists," Jews speaking only Greek, possibly diaspora Jews who had resettled in Jerusalem. With the start of their missionary activity, early Jewish Christians also started to attract proselytes, Gentiles who were fully or partly converted to Judaism. According to James Dunn, Paul's initial persecution of Christians probably was directed against these Greek-speaking "Hellenists" due to their anti-Temple attitude. Within the early Jewish Christian community, this also set them apart from the "Hebrews" and their Tabernacle observance.

Christian missionary activity spread "the Christian Way" and slowly created early centers of Christianity with Gentile adherents in the predominantly Greek-speaking eastern half of the Roman Empire, and then throughout the Hellenistic world and beyond the Roman Empire in Assyria, Mesopotamia, Armenia, Georgia and Persia. Early Christian beliefs were proclaimed in kerygma (preaching), some of which are preserved in Christian Testament scripture. The early Gospel message spread orally, probably originally in Aramaic, but almost immediately also in Greek.