19 “O God, You have taught me from my youth”. What have you taught me? That of Your righteousness alone I ought to be mindful. For reviewing my past life, I see what was owing to me, and what I have received instead of that which was owing to me. There was owing punishment, there has been paid grace: there was owing hell, there has been given life eternal. “O God, You have taught me from my youth.” From the very beginning of my faith, wherewith You have renewed me, You taught me that nothing had preceded in me, whence I might say that there was owing to me what You have given.
For who is turned to God save from iniquity? Who is redeemed save from captivity? But who can say that unjust was his captivity, when he forsook his Captain and fell off to the deserter? God is for our Captain, the devil a deserter: the Captain gave a commandment, the deserter suggested guile: where were your ears between precept and deceit? Was the devil better than God? Better he that revolted than He that made you? You believed what the devil promised, and found what God threatened.
Now then out of captivity being delivered, still however in hope, not yet in substance, walking by faith, not yet by sight, “O God,” he says, “You have taught me from my youth.” From the time that I have been turned to You, renewed by You who had been made by You, re-created who had been created, re-formed who had been formed: from the time that I have been converted, I have learned that no merits of mine have preceded, but that Your grace has come to me gratis, in order that I might be mindful of Your righteousness alone.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)