Paul set the tone for Christianity, including emphasis on celibacy and theory of divine grace and salvation, as well as eliminating the circumcision requirement. It was Paul who called the New Testament euangelion, 'the gospel'.
Paul met James, the brother of Jesus, and Peter, the Apostle, in Jerusalem. He then went on to Antioch where he converted Gentiles. This helped make Christianity a universal religion.
Paul of Tarsus is arguably a secondary founder of Christianity - after Jesus - since two-thirds of the New Testament contains Paul's writings. Paul's story can be found mostly in the Book of Acts in the New Testament, but additional bits and pieces appear in parts of his letters to the Galatians, the Corinthians and others.
Paul's name originally was Saul and he was from the city of Tarsus in what is now the southern part of Turkey, but was then called Asia Minor. He was Jewish and as the movement around Jesus began to grow, especially after Jesus' death, Saul began to try to root the so-called "Christians" out of the Jewish community. The Jews at this time were living under Roman rule and sporadically endured persecution for their faith. Many feared that the Jesus movement and its somewhat radical and "new" claims about the "kingdom of God" would draw unwanted attention from the Roman authorities and would create difficulties for the Jewish community. So, Saul traveled around attempting to root out the Jesus movement from the Jewish synagogues.
According to the story in the Book of Acts, Saul has a mystical experience on the road to Damascus in Syria after which he ironically converted to the very faith he had been trying to root out. While walking on the road, he went blind, heard a divine voice, and was given instructions that when followed put him in touch with certain Christians who welcomed him even though he was their enemy of sorts. After this experience, Saul converted to the Christian belief and his named was changed to Paul....