Acts 23:1-2 KJV
And Paul, earnestly beholding the council, said, Men and brethren, I have lived in all good conscience before God until this day.
And the high priest Ananias commanded them that stood by him to smite him on the mouth.
Paul's offense, and the one for which he was here smitten, was claiming to have lived in good conscience before God. The high Priest would have known his history. He would have known that Paul had been a Pharisee but that he had turned from that to follow Christ.
Paul was judged for becoming a follower of the Lord. Though his conscience was clear the High Priest's couldn't have been. He was leader of the religion that had crucified Christ. His seared conscience could not tolerate the good conscience of a Christian.
The history of the Christian faith is stained with the blood of men and women whose consciences were clear with God but whose testimony inflamed the consciences of others.