Early Christianity is commonly known as the Christianity of the roughly three centuries (1st, 2nd, 3rd, early 4th) between the Crucifixion of Jesus (c.26-36) and the First Council of Nicaea in 325.

At first, the Christian church was centered in Jerusalem, in an "upper room" perhaps where the Cenacle is today, and leaders included James, Peter, and John. The major primary source for the Apostolic Age (c.30-c.100) is the Acts of the Apostles, but its historical accuracy is disputed. Following the Great Commission, the missionary activity of the Apostles, including Paul of Tarsus, spread Christianity to cities throughout the Hellenistic world, such as Alexandria and Antioch, and also to Rome and even beyond the Roman Empire. The term "Christian" was first applied to members of the church at Antioch according to Acts 11:26. The New Testament includes letters written by Paul to churches, such as those in Thessalonica and Corinth, during the years 50-62, see also Seven Churches of Asia. Early Christians continued the Jewish practice of reverence for the "Jewish Scriptures", using the Septuagint translation of Hellenistic Judaism that was in general use among Koine Greek-speakers, or the Targums in use among Aramaic-speakers, but added to it their own writings.

In 70 the Second Temple was destroyed, and in c. 135 Jews were expelled from the renamed pagan city after the Bar Kokhba revolt. Among those who left the city were most of the Christian population. Following this time, early Church historian Eusebius of Caesarea records that ethnically Jewish leadership of the church in Jerusalem (literally those "of the circumcision") was replaced by Gentile leadership. According to the Catholic Encyclopedia these were Suffragan bishops appointed by the Metropolitan bishops of the capital in Caesarea. Similarly, Claudius had expelled the Jews from Rome in 49, though Nero allowed their return but turned against "Christians" (according to Tacitus) after the Great Fire of Rome of 64, the beginning of persecution by Roman authorities. Historians debate whether or not the Roman government distinguished between Christians and Jews prior to Nerva's modification of the Fiscus Judaicus in 96. From then on, practising Jews paid the tax, Christians did not.

Christianity spread further during the 2nd century. Notable proto-orthodox leaders and writers of this time include Irenaeus of Lyon, Polycarp of Smyrna, Ignatius of Antioch, Clement of Rome, and Justin Martyr. The 2nd century was also the time of several who were later declared to be major heretics, such as Marcion, Valentinius, and Montanus. During the third century, Christianity further increased in numbers (Robin Lane Fox suggests that Christians composed about 2% of the Empire by 250). Teachers of this period, including Origen in Alexandria and Tertullian in North Africa, expressed in their writings doctrines seen as Trinitarian. Anthony the Great and others established Christian monasticism, and Gregory the Illuminator was responsible for Armenia becoming the first officially Christian country. Following the conversion of Constantine the Great (just prior to the Battle of the Milvian Bridge in 312), the Roman Empire tolerated Christianity with the Edict of Milan in 313, leading later to the adoption of Christianity as the state religion in 380 by Theodosius I (see Edict of Thessalonica) and the rise of Christendom in the Byzantine empire.

What started as a religious movement within first century Judaism therefore became, by the end of this period, the favored religion of the Roman Empire, as well as a significant religion outside the empire. According to Will Durant, the Christian Church prevailed over Paganism because it offered a much more attractive doctrine and because the church leaders addressed human needs better than their rivals. The First Council of Nicaea marks the end of this era and the beginning of the period of the first seven Ecumenical Councils (325 - 787).