If you read through the book of Acts, you’re introduced to a man by the name of Saul of Tarsus in Acts 7:58, where he appears during the execution of Stephen. In Saul’s zeal, he sought to exterminate all believers of “The Way”—that is, Christians. “Saul was ravaging the church, and entering house after house, he dragged off men and women and committed them to prison (Acts 8:3).
Christ transformed Saul in Acts 9, then after a number of episodes with the Apostle Peter (Acts 10-12), Saul reemerges with his friend, Barnabus, in Acts 12:25:
And Barnabus and Saul returned from Jerusalem when they had completed their service, bringing with them John, whose other name was Mark.
Soon, the church at Antioch commissioned them by the Holy Spirit, were sent off to minister to the Gentiles, to which Christ called him as well (Acts 9:15-16).
It’s in the next scene that we see an interesting verse: “But Saul, who was also called Paul . . .” (Acts 13:9).
I’ve heard over the years that Saul was his pre-Christian name, but Paul served as his ‘Christian’ name. Yet, Luke (the author of Acts) used Saul’s name after his conversion—so much for that theory.
The truth is, since Saul was a Roman citizen, he possessed a Roman name as well—Paul. So, Paul used this name for his ministry to the Gentiles, who would receive him
So, for what it’s worth, that’s the short answer. Paul is not his ‘Christian’ name, but his Roman name. He was still ‘Saul’ when he returned to synagogue in his hometown.