Most of us know him as the apostle Paul, but he was using the name Saul when we first encounter him in scripture as a zealous Pharisee who was persecuting the church (Acts 9:1-2). It’s obvious that he didn’t believe that Jesus was the Christ. But Jesus appeared to Saul while he was traveling to Damascus to wreak more havoc on the disciples, and he became a believer in the Lord, was baptized, and accepted the commission which Jesus gave to him - to preach the gospel to the Gentiles (non-Jews).
Some people have suggested, and you may have heard it taught that Saul’s name was changed to Paul as a result of his conversion. Is this true? What do we know? Here’s what we know as a fact: “But Saul, who was also known as Paul...” (Acts 13:9). That’s it! Do we know how it came about that he had these two names? No. But here are some possibilities.
First: Jesus may have given him the name Paul after his conversion. Some have said this. Jesus gave the name “Cephas” (Peter) to Simon (John 1:42). But there is nothing in scripture to support that Jesus did a similar thing with Saul.
Second: Saul may have had both names from birth. Saul is very much a Hebrew name, and Paul is more Greek/Roman. Saul was born a Roman citizen (Acts 22:28), so it would be no surprise that he always had these two names. This presents two possibilities:
1) He chose to use the name Paul after his conversion to disassociate himself from his former life when he was known as Saul the one who persecuted the church.
2) He chose to use his Greek/Roman name when he began his mission among the Gentiles to be more readily heard and accepted among them.
Whatever the case was (and it may have been something else entirely), we don’t know what caused Saul to start using the name Paul. We only know that he did.