In reading the book of Acts in the Bible we learn about a badly misguided dude named Saul who turns his life around and later goes by the name of Paul. When did this name change take place?
If you say his name changed when he stopped killing Jesus’ followers to become one himself, I’d agree with you — and we’d be wrong.
It’s subtle, but Acts 13:9 says, “Saul, who was also called Paul, filled with the Holy Spirit…” I can’t say for sure, but this seems to be the first indication of God’s spirit filling Saul. Sure, he was a follower of Jesus before that and he was doing things for God before that, but when the Holy Spirit filled him, it’s as if God gave him a new name, Paul.
The change is abrupt. In Acts 7:58 through Acts 9:13 he is Saul. The Holy Spirit shows up in Acts 9:13 and for the rest of the book of Acts (and the rest of the Bible) he goes by Paul.
And that’s when his ministry took off.