Christianity is in a direct sense an offshoot of Judaism, because Jesus and his immediate followers were Jews living in Palestine and Jesus was believed by his followers to have fulfilled the Old Testament prophecies of the Messiah. Following a trend of proselytization in the Judaism of that period Christianity was from its beginnings expansionist. Its early missionaries (the most notable of whom was St. Paul, who was also responsible for the formulation of elements of Christian doctrine) spread its teachings in Asia Minor, Alexandria, Greece, and Rome. Missions have remained a major element in Christianity to the present day.
For the first three centuries of Christianity, history is dependent on apologetic and religious writings; there are no chronicles (see patristic literature). Historians differ greatly on how far back the 4th-century picture of the church (which is quite clear) can be projected, especially respecting organization by bishops (each bishop a monarch in the church of his city), celebration of a liturgy entailing a sacrament and a sacrifice, and claims by the bishop of Rome to be head of all the churches (see papacy). There is evidence for these features in the 2d cent. A first problem for Christians was how to resist attempts to interpret the new beliefs in pagan terms (e.g., Gnosticism). The earliest sectarian deviations were those of Marcion and of Montanus (2d cent.). They were handled resolutely by the church; the teachers of novelty were expelled (excommunicated).
For 250 years it was a martyrs' church; the persecutions were fueled by the refusal of Christians to worship the state and the Roman emperor. There were persecutions under Nero, Domitian, Trajan and the other Antonines, Maximin, Decius, Valerian, and Diocletian and Galerius; Decius ordered the first official persecution in 250. In 313, Constantine I and Licinius announced toleration of Christianity in the Edict of Milan. In the East the church passed from persecution directly to imperial control (caesaropapism), inaugurated by Constantine, enshrined later in Justinian's laws, and always a problem for the Orthodox churches. In the West the church remained independent because of the weakness of the emperor and the well-established authority of the bishop of Rome.
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