Atti degli Apostoli 13:9

Then Saul, who also is Paul. This is the first time we find the apostle called Paul. Some, therefore, think it was given him when he converted this proconsul, Sergius Paulus. Others, that Saul being a Hebrew word, the Greeks, or rather the Romans, turned it into Paul. (Witham) --- This is the first place in which this apostle is called Paul. He took this name out of respect to the illustrious convert he had made in the person of the proconsul, the governor of the island.

(Menochius) --- Or, more probably, his former name, by a small change, was modelled into Paulus, which was a sound more adapted to a Roman ear. He begins to bear this name only, when he enters on his mission to the Gentiles. (Calmet)