18 With reason there follows, “I will enter into the power of the Lord:” not my own, but the Lord's. For they gloried in their own power of the letter, therefore grace joined to the letter they knew not....But because “the letter kills, but the Spirit makes alive:” “I have not known literature, and I will enter into the power of the Lord.” Therefore this verse following does strengthen and perfect the sense, so as to fix it in the hearts of men, and not suffer any other interpretation to steal in from any quarter.
“O Lord, I will be mindful of Your righteousness alone”. Ah! “alone.” Why has he added “alone,” I ask you? It would suffice to say, “I will be mindful of Your righteousness.” “alone,” he says, entirely: there of my own I think not. “For what have you which you have not received? But if also you have received, why do you glory as if you have not received.” Your righteousness alone does deliver me, what is my own alone is nought but sins. May I not glory then of my own strength, may I not remain in the letter; may I reject “literature,” that is, men glorying of the letter, and on their own strength perversely, like men frantic, relying: may I reject such men, may I enter into the power of the Lord, so that when I am weak, then I may be mighty; in order that You in me may be mighty, for, “I will be mindful of Your righteousness alone.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)