In Acts 13:9, we first learn that Saul is also Paul. The circumstances for the new name are curious, Paul is first referred to as “Paul” in connection with the story of Sergius Paulus:
7 He was with the proconsul, Sergius Paulus, a man of intelligence, who summoned Barnabas and Saul and sought to hear the word of God.
8 But Elymas the magician (for that is the meaning of his name) opposed them, seeking to turn the proconsul away from the faith.
9 But Saul, who was also called Paul, filled with the Holy Spirit, looked intently at him
10 and said, “You son of the devil, you enemy of all righteousness, full of all deceit and villainy, will you not stop making crooked the straight paths of the Lord?:
Witherington, in his excellent commentary on Acts, argues that Saul’s name as “Paul” “probably rules out the idea” that Paul’s name has something to do with Paulus. I’ll let Witherington speak in a moment. I would merely add that the comment “who was also called Paul” does not necessary tell us when Saul was first alternatively known as Paul. Luke’s language is as ambiguous as could be on the point in question, there is no verb in the phrase, “Σαῦλος δέ, ὁ καὶ Παῦλος ” (But Saul, the and/also Paul — in a painfully wooden translation; better, “Saul who is also Paul”). Now Saul could also been known as Paul since birth. But if that is so, why did Luke wait until this point to raise the matter?
Paul Barnett in Jesus & The Rise of Early Christianity writes, “Moreover, Paul may have changed his name at this juncture, not only to mark the conversion of Serguis Paulus, the first Roman governor to embrace the faith of Christ, but also to acknowledge the patronage and protection of the Paulii family in Pisdia” (279).
In addition, this was not the firs time Saul/Paul would have interacted with Gentiles (Peter had already opened a mission to the Gentiles. In fact the name is noted in connection with Paul’s interaction with a Jew.). The point is not definitive in any direction.
With those questions (I am agnostic on the topic), Witherington writes:
The second name conundrum arises at v. 9, where for the first time we hear of Saul also being called Paul. As Barrett astutely puts it, this does not represent a change in name, but the identification of an alternative name.168 Luke’s way of putting it probably rules out the idea that Paul borrowed the name of the proconsul. It is probably right to say that Luke has introduced the name at this juncture because now Paul will be dealing with Gentiles and will accordingly want to use his Roman name in doing so. Presumably Paulus was the apostle’s cognomen, though it may have been his praenomen or even a nickname or supernomen, for “paulus” in Latin means little.169
fn 168: Cf. Ramsay, St. Paul the Traveller, p. 81; Barrett, Acts, vol. 1, p. 616. As Ramsay suggests, these alternative names would be used according to what audience Paul would be addressing, or, more to the point, according to what locale he was visiting. Part and parcel of a κατα γενος approach to history writing meant one was sensitive to the nuances involved in a change of ethnic or geographical setting. Luke is signaling that henceforth Paul would primarily be in regions where his Greek (or Roman) name would be apropos.
fn. 169: Acts 13:9 surely has the same intent as in 12:12, where an alternative name is in view. Cf. Leary, “Paul’s Improper Name”; Hemer, “The Name of Paul”; and my Conflict and Community in Corinth, p. 5 and n. 12. Leary notes a reason why he would not have wanted to be called “saulos” in the Greco-Roman world, a word that refers to the wanton way of walking of prostitutes. Only Luke tells us Paul’s Semitic name, but some possible confirmation that it is correct is that Paul says he was of the tribe of Benjamin (Phil. 3:5).