He tells us exactly what he was like as a Pharisee. I counted it all out and it was gain. Stock of Israel, Hebrew of Hebrews, etc. He fueled his spirit with this adoration of his good works. It produced the stoning of Stephen.
Religion is always one step away from murder.
Think about that. Filled with hatred he persecuted the church thinking he was serving God. Then Christ comes to Him on the Damascus road. Paul now realizes that he is accepted not by good works that he has done, but by the finished work of Jesus Christ on the cross. He is swept up - pardoned and declared righteous and carried into God family.