How did Paul have authority to imprison Jewish believers before his conversion?
Paul acted under authority received from the chief priests. Rome respected Judaism as an ancient religion and allowed Jewish leaders to exercise internal jurisdiction over their own people, even imprisoning and putting them to death. So Paul could imprison Jewish believers as heretics, something Rome would not permit against a Gentile.
Les teaches that Paul himself acknowledged this, saying he shut up many of the saints in prison after receiving authority from the chief priests, and that he voted to put them to death. Les explains that the Jews maintained this kind of authority in order to keep their temple going. Rome condescended to Judaism, even guaranteeing safe delivery of Jewish offerings sent to the temple in Jerusalem, and gave them permission to put their own people to death. That is why Paul could imprison Jewish believers, and where was Rome? Rome would never have allowed that to happen to a Gentile. Les points out that the Romans had respect for ancient religions, and Judaism was an ancient religion. But when Christianity appeared under the Roman Empire, it was not an ancient religion, so Rome tried every way to stamp it out, and Christians came under massive persecution while the Jews did not.