Paul, who was originally called "Saul of Tarsus", was one of the most important leaders of the "Apostolic Age" in the first century A.D. Though he dedicated the early part of his life to persecuting followers of Jesus Christ, he later spent most of his life teaching the Gospel of Christ.
Paul the Apostle, though not considered one of the original 12 apostles of Christ, is considered one of the most important figures of the early church. Out of the 27 books in the New Testament, Paul is credited as the author of 13 of them.
It is said that the resurrected Christ appeared to Paul, and this caused him to convert to Christianity and begin to preach that Jesus was the Son of God. Paul traveled far and wide, evangelizing the life and Gospel of Christ, which he asserted that he received not from man but directly as "the revelation from Jesus Christ."
The core of Paul the Apostle's teaching revolves around the idea that Jesus died so that the sins of his believers would be forgiven because of their faith. For his work with missionaries and the resultant spread of the Christian faith, Paul was martyred by the Romans.