“My manner of life then from my youth up, which was from the beginning among mine own nation and at Jerusalem, know all the Jews, having knowledge of me from the first, if they be willing to testify, that after the most strict sect of our religion I lived a Pharisee.”
He first declares that all who knew him could testify of the fact that he had lived strictly and honestly as a Pharisee, that is (for the Gentiles among his hearers) as one of the strictest adherents of Judaism. This would impress any Caesarean Jews present, for all would know of the dedication of the Pharisees, and it would assure the Gentiles present that he had lived in a godly fashion. He was making all know the piety of his life up to that point. And the point was that what a man was he mostly remained. His views may change but not his approach to life.
The Hope of the Coming Messiah and of the Resurrection