In verses 13 and 14, Paul talks about his great zeal in religious matters prior to his coming to faith in Christ. He was very well known. Everyone had heard about him— especially the Christians! His one ambition in life was to persecute the Church of God and try to destroy it.
It must be understood that Paul was a very religious man. He was very sincere. He was highly educated. He outshone many of his Jewish friends in his zeal for God. The Jewish leaders greatly respected him.
The trouble was: he was wrong! He did not know the truth. He did not know God. He only knew about Him. He made the fatal mistake of thinking the two were one and the same. He thought that knowing about God was the same as knowing Him.
You might stop and ask yourself, do you really know God, or do you only know about Him? Lots of people in the world today, especially religious people, have made the same blunder as Paul. Their belief in God is an intellectual one. It is a theoretical faith, not a living, personal one.
Such a belief in God is utterly useless, as Paul-discovered. (Many years later, Paul wrote about his zealous fellow Jews who had still not come to faith in Christ. Read about it in Romans 10:1-4.)