Born around the beginning of the Common Era, Paul was a Hellenistic Jew who, according to tradition, was a Pharisee and may have been a member of the Sanhedrin. He was responsible for working against the development of Christianity but, according to his own accounts, he heard the voice of Jesus and was struck blind while on his way to Damascus.
After this time, he became an ardent defender and promoter of Christianity, even taking for himself the title "apostle" and eventually became responsible for many of the books in the New Testament. Some lists of the apostles apparently include Paul as the twelfth apostle, replacing Matthias. Since Paul asserted the authority to be an apostle to the Gentiles, though, he is more appropriately listed as the thirteenth because for Christian symbolism to work there needs to be twelve apostles representing the original twelve Hebrew tribes and then a thirteenth for non-Jews.
Paul insisted on using the title apostle despite never having met Jesus and never being called personally by Jesus because of his vision on the road to Damascus. Paul claimed that the resurrected Jesus spoke to him and called him at that time. Taking the title "apostle" has clear political implications because the original apostles would have had the greatest authority within the growing Christian movement. They were, after all, the ones who had personally known and personally been called by Jesus. This would have put Paul in a secondary position at best, especially given his recent background as an opponent of Christianity.
Adopting the title apostle could have been perceived as an attempt to assert equal authority alongside the original apostles, especially since Paul claimed to have been called by the resurrected Jesus rather than by Jesus when he was still alive. On more than one occasion in his letters Paul has to defend his status as an apostle, suggesting that it was questioned by various people.