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The starting point for the Christianization of the Roman Empire was Jerusalem. The disciplines of Christ fled to Galilee after his Crucifixion but soon returned to Jerusalem, where the community of the Nazarenes counted some 8,000 members not long thereafter. While James, one of Christ's Apostles, led this community, Peter pursued missionary activities in communities in Syria. John spread the word of Christ in Ephesus, and Philipp sought converts in Samaria and Caesarea. Peter is reported to have visited Rome as early as 42 AD and contributed to the development of a community of the faithful there, although there is no historical evidence that this journey actually took place.
Certainly the most influential Apostle after Peter was Paul, who undertook three major missionary expeditions through Asia Minor, Greece and Macedonia between 45 and 58 AD. Repeated conflicts with conservative Jews regarding the issue of circumcision eventually culminated in his arrest and extradition to Rome, where he is presumed to have died in the course of Nero's campaign of persecution waged against Christians.

Progress following the death of the Apostles
During the first century AD, the Christian faith took root in Palestine, Syria, Asia Minor — with significant concentrations in the provinces of Lykaonia, Pisidia, Phrygia and Asia, in several parts of Macedonia and Greece, in Rome itself and in the Italian port city of Puteoli (the present Pozzuoli). Christianity reached the fringe regions of the Middle East and Asia Minor, the Balkan Peninsula, Gaul, Spain and North Africa (Carthage) during the second century. Asia Minor, where most of the early communities formed, was the first region in which Christianity became the dominant religion.
Widespread persecution of Christians did not begin until the middle of the third century, although isolated anti-Christian campaigns were undertaken during Nero's reign, and Christians were martyred in Lyon in 177 AD. It was not until every citizen of the Empire was required to make sacrifices to the old gods during the crisis of the third century that Christians, who could not reconcile such offerings with their beliefs, were persecuted on a large scale. The edicts of Valerian of 257 imposed severe penalties on professed Christians, including confiscation of property, banishment and execution. After Gallienus put an end to this wave of persecution in 259, the Church experienced a period of peace.
The edicts of Diocletian, which were part of his campaign to rebuild the Roman state at all levels, unleashed the last and most extensive wave of persecution. Christians were stripped of their offices, honours and legal status, and ordered to make offerings on penalty of death. The Edict of Toleration issued by Galerius in 311 put an end to the persecution of Christians and restored their former rights. Christianity spread progressively through much of Western and Southern Europe, North Africa, Asia Minor and the Middle East during the course of this century and the early years of the next.
The groundwork for replacement of the Roman state religion by Christianity was laid during the reign of Constantine the Great, and the process was completed by the Eastern Roman Emperor Theodosius in 380 AD. The Christian faith was made mandatory for all citizens of the Empire, and Christianity became the official state religion. As the imperial church, the Church, which was endangered by disputes over dogma and sectarian dissent, not only maintained its unity but became an important guardian of the Roman heritage in the face of increasing barbarization and regionalization in consequence of the migration of Germanic peoples in the fifth century.
K. Lückemeier, E. Astor; Ü: J. Southard