Christianity Without Paul
What would the religion be like if the Apostle Paul had never lived?
Paul, however, transformed this proclamation of Jesus to the proclamation about Jesus, teaching that what really mattered for a human's relationship with God was not repentance from sin (Jesus' own emphasis), but the death and resurrection of Jesus himself. Anyone who trusted Christ's death and accepted the fact of his resurrection would be right with God, and so would enter into God's kingdom when it arrived. And most important, this was true of all people, whether Jew or Gentile. For Paul--and this was a radical teaching at the time, even if it seems commonplace today--a person did not have to become a Jew in order to be a Christian. For in Christ, God's promises are fulfilled to all people, both Jew and Gentile, on equal terms; it is through his death that people are reconciled with God.
Third, with respect to the spread of Christianity:
without Paul, the Christian mission, which eventually overtook the entire Roman empire, may never have happened as it did. As one of the earliest, the best known, and arguably the most effective of early Christian missionaries, Paul established churches in key urban areas of the northern Mediterranean, especially in Asia Minor (modern Turkey), Macedonia, and Achaia (modern Greece). These churches then grew and spread, leading to the Christianization of many of the provinces of the Roman Empire. It took several centuries, but eventually this new religion became the official religion of the Empire itself.
What is particularly important is that for Paul, this Christian mission was to go not simply to Jews scattered throughout the world, but to both Jews and Gentiles. And in fact, in Paul's churches, most of the converts were Gentile--former pagans (one-time adherents of the various polytheistic religions of the Roman world). Within a generation or so of Paul's death, the vast majority of all converts were from the ranks of paganism. Had this shift from Jew to Gentile never happened, arguably the conversion of the Roman empire would never have taken place, since Christianity would have remained a form of Judaism, not a religion open to all peoples.