[Paul] says, therefore: Whatever gain I had, i.e., prestige, namely, to be a Pharisee and so on, I counted as loss for the sake of Christ, i.e., I came to regard them as hindrances. For the observances of the Law, which were effective during the time of the Law, became harmful after Christ; hence he says, loss. And the reason for abandoning them was Christ; hence he says, for the sake of Christ. He explains this: first, that he acted thus in order to know Christ, and secondly to obtain Him. In regard to the first he says, Indeed I count everything as loss. This is true, if he had continued to depend on them. What I did formerly, I now regard a loss on account of my desire for a correct understanding of Christ, my Lord: “For I decided to know nothing among you except Jesus Christ and him crucified” (1 Cor. 2:2). Because of the surpassing worth of knowing Christ Jesus my Lord, since this transcends all knowledge. For there is nothing better to be known than the Word of God in whom are hid all the treasures of wisdom and knowledge” (Col. 2:3).
Thomas Aquinas, Commentary on St. Paul’s Letter to the Philippians.