From Catholic Answers:
Saul of Tarsus was born a Jew, "circumcised on the eight day, of the race of Israel, or the tribe of Benjamin, a Hebrew of Hebrew parentage, in observance of the law a Pharisee" (Phil 3:5). The Hebrew name given him by his parents was Saul, but, because his father was a Roman citizen (and therefore Saul inherited Roman citizenship), Saul also had the Latin name Paul (Acts 16:37, 22:25-28), the custom of dual names being common in those days. Since he grew up in a strict Pharisee environment, the name Saul was by far the more appropriate name to go by. But after his conversion Saul determined to bring the gospel to the Gentiles, so he dusted off his Roman name and became known as Paul, a name Gentiles were accustomed to.
I wonder if he was named Saul after King Saul, the standout success (practically the only one!) of the tribe of Benjamin. Were boys in the tribe of Judah ever named “Saul,” in that time period?
A Salvadoran family in our congregation recently named their son “Saulo.” Family name, attachment to King Saul (I think he got an undeserved bad press), thinking of St. Paul but already had a “Pablo”? I didn’t ask ;-).