13 For you have heard of my former way of life in Judaism, how severely I persecuted the church of God and tried to destroy it. -Galatians 1:13 BSB
Before Paul’s conversion to Christianity, he was active, as a Pharisee (see the commentary on verse 11), in persecuting the church of the Lord for the sole purpose of destroying it. He thought that he was doing God’s work, see John 16:2, but in reality, he was doing Satan’s work—trampling the gospel as much as possible so it won’t spread to the surrounding areas by putting the saints in prison.
Saul, who was also known as Paul, fought against God’s church
3 But Saul began to destroy the church. Going from house to house, he dragged off men and women and put them in prison. -Acts 8:3 BSB
1 Meanwhile, Saul was still breathing out murderous threats against the disciples of the Lord. He approached the high priest 2 and requested letters to the synagogues in Damascus, so that if he found any men or women belonging to the Way, he could bring them as prisoners to Jerusalem. -Acts 9:1-2 BSB