What did Paul have to recognize about his past persecution before God could use him?
Paul had to recognize that his past persecution of Jewish believers—doing many things contrary to the name of Jesus of Nazareth—was what he had to go through before God could use him. He openly took responsibility for those acts, acknowledging he had done them himself in Jerusalem.
Les explains that Paul rehearses this in the first person in Acts 26. In verse 9 Paul says he verily thought with himself that he ought to do many things contrary to the name of Jesus of Nazareth. Les points out that Paul never got over it, but nevertheless he had to recognize that this was what he had to go through before God could use him. In verse 10 Paul says he did these things in Jerusalem, and many of the saints, these Jewish believers, did he persecute. He takes responsibility for it himself, showing he had to be a pretty big wheel in Judaism to have that kind of authority.
I verily thought with myself, that I ought to do many things contrary to the name of Jesus of Nazareth.