He knew legalism to the core. He understood its fallacies and weaknesses. When it came to the laws of Judaism, Paul kept them religiously. He was confident in his religious regulations. Now he is confident in his religion.
Religion is a false basis of confidence because it rests upon the accomplishment of man.
A Pharisee went to the synagogue three times a day. He prayed seven times a day. Christianity rests upon the accomplishments of Christ. No one is ever right with God by logging a great amount of time in religion. Religion attacks the grace principle as they did Paul.