One important reason for the spread of Christianity was the evangelical missions of Paul of Tarsus. Paul, originally called Saul, was perhaps the one person most responsible for the spread of Christianity throughout the Roman Empire. Paul was not one of the original 12 Apostles of Jesus Christ, whose life and teachings are the basis of the Christian religion. Paul originally persecuted the followers of the teachings of Jesus until he had a conversion experience after which he became a strong follower of Christ and began his evangelical missions. Paul, a Greek speaking Jew from Asia Minor, went on three missions covering 10,000 miles over the course of thirty years. He traveled through Palestine, Syria, Asia Minor, Macedonia, Greece, and Rome, establishing Christian communities in each area. Paul made the decision to spread the Christian faith to non-Jews, which helped establish the Christian religion throughout the Roman Empire. The Roman system of roads allowed the Christian message to travel quickly from one place to another. The fact that Paul spoke Greek and was a Roman citizen also helped Paul in his travels to spread the Christian religion.
Several factors allowed Christianity to spread within the Roman Empire.
The main cause of its spread was that it was an evangelical religion, sending missionaries across the Roman Empire. The decision, made first by Peter and then expanded upon by Paul, to preach to gentiles as well as Jews and not to require gentile converts to follow Jewish law increased its appeal.
The Pax Romana, as well as extensive road and shipping network of the Roman Empire, facilitated the physical circulation of missionaries and letters, and thus was another enabling condition for the spread of Christianity.
Finally, polytheism was a system in which new gods were normally introduced freely and their worship allowed. Where Christianity, as Judaism before it, ran into trouble was not in worshipping a new God, but in insisting that this was the only God and refusing to worship the state gods and deified emperor, an act that was viewed as a form of treason, until Constantine who legalized Christianity.