Which church was Paul persecuting when he said he persecuted the church of God?
The church Paul persecuted was the Church of God at Jerusalem. The context connects that persecution to the Jerusalem church, where Stephen was stoned and Saul consented to his death, and where Jews who had embraced Jesus of Nazareth as Messiah were targeted.
Les explains that Paul's persecution was against the church at Jerusalem, identified as the Church of God. In Acts 7 Stephen is stoned in 36 A.D., seven years after the crucifixion placed at 29 A.D. Acts 8:1 says Saul was consenting to Stephen's death. Les notes Paul later wrote that it plagued him throughout his remaining life that he had persecuted the Church of God. He was the leader of this terrible persecution to stamp out Jews who had embraced Jesus of Nazareth as Messiah. Acts 22 and 26 rehearse his conversion on the road to Damascus, where the Lord struck and converted him, bringing him to realize Jesus was indeed the Christ.