17 “My God, the mercy of Him shall come before me”. Behold what is, “My strength, to You I will keep:” on myself I will in no ways at all rely. For what good thing have I brought, that you should have mercy on me, and should justify me? What in me have You found, save sins alone? Of Yours there is nothing else but the nature which Thou hast created: the other things are my own evil things which You have blotted out. I have not first risen up to You, but to awake me You have come: for “His mercy shall come before me.” Before that anything of good I shall do, “His mercy shall come before me.” What answer here shall the unhappy Pelagius make? “My God has shown to me among mine enemies”. How great mercy He has put forth concerning me, among mine enemies He has showed. Let one gathered compare himself with men forsaken, and one elect with men rejected: let the vessel of mercy compare itself with the vessels of wrath; and let it see how out of one lump God has made one vessel unto honour, another unto dishonour.
“For so God, willing to show wrath, and to manifest His power, has brought in, in much patience, the vessels of wrath, which have been perfected unto perdition.” And wherefore this? “In order that He might make known His riches upon the vessels of mercy.” If therefore vessels of wrath He has brought in, wherein He might make known His riches upon the vessels of mercy, most rightly has been said, “His mercy shall come before me: My God has showed to me among mine enemies:” that is however great mercy He has had concerning me, to me He has showed it among these men concerning whom He has not had mercy. For unless the debtor be in suspense, he is less grateful to him by whom the debt has been forgiven. “My God has showed to me among mine enemies.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)