He knew it all. He was at the top of the religious game. He, of course, had faith, belief, action, morality, and church attendance. He was as spiritual as you could ever hope to be. I assume he never once entertained the thought that he needed any further conversion.
And yet, he was persecuting believers of a new faith - a sure sign you are always on the wrong track.
The story says he was blinded by God - physically illustrating what was already true.
If you think you know it all - and are persecuting (in any way) fellow humans - beware.