12 “I will confess to You for ever, because You have done”. “Have done what?” Doeg You have condemned, David You have crowned. “I will confess to You for ever, because You have done.” Great confession, “Because you have done”! “Have done” what? Except these very things which above have been spoken of, that like an olive fruit-bearing in the house of God, I should trust in the mercy of God for everlasting and world without end? You have done: an ungodly man cannot justify himself.
But who is He that justifies? “Believing,” he says, “on Him” that justifies “the ungodly.” “For what have you which you have not received? But if you have received, why do you glory as if you have not received, as if of yourself you have?” Be it far from me that I should so glory, says he, that is opposed against Doeg, that bears with Doeg upon earth, until he remove from his dwelling, and be rooted up from the land of the living. I glory not as if I have not received, but in God I glory.
“And I will confess to You because You have done,” that is, because You have done not according to my merits, but according to Your mercy. But I have done what? If you recollect, “Before, I was a blasphemer, and a persecutor, and injurious.” But you, what have you done? “But mercy I have obtained, because ignorant I did it.” “I will confess to You for ever, because You have done.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)