Wikipedia describes St. Paul as an Apostle, "though not one of the Twelve Apostles". It goes on to say that "It has been popularly assumed that Saul's name was changed when he converted from Judaism to Christianity, but that is not the case."
To paraphrase Wikipedia: his Jewish name was Saul (meaning "asked for, prayed for, borrowed"), possibly after the Old Testament King Saul. But he inherited Roman citizenship from his father, and his Latin name was Paul. It was quite usual for the Jews of that time to have two names – one Hebrew, the other Latin or Greek.
According to the Book of Acts, Jesus called him Saul, "in the Hebrew tongue". God referred to him as "Saul, of Tarsus". The first time he is referred to as Paul is when he's on the island of Cyprus – long after his conversion; the author (Luke) indicates that the names were interchangeable: "Saul, who also is called Paul." Thereafter he is always referred to as Paul, not least in the books that he wrote himself.
Wikipedia notes that Paul's use of his Roman name was "typical of [his] missionary style. His method was to put people at their ease and to approach them with his message in a language and style to which they could relate."
© Haydn Thompson 2017