10 Listen to the Apostle Paul acknowledging grace, and afterwards desiring the payment of a debt. What acknowledgment of grace is there in Paul? “Who was before a blasphemer, and a persecutor, and injurious; but I obtained,” says he, “mercy.” He said that he who obtained it was unworthy; that he had, however, obtained it, not through his own merits, but through the mercy of God. Listen to him now demanding the payment of a debt, who had first received unmerited grace: “For,” says he, “I am now ready to be offered up, and the time of my departure is at hand.
I have fought a good fight, I have finished my course, I have kept the faith: henceforth there is laid up for me a crown of righteousness.” Now he demands a debt, he exacts what is due. For consider the following words: “Which the Lord, the righteous Judge, shall render unto me in that day.” That he might in the former instance receive grace, he stood in need of a merciful Father; for the reward of grace, of a just judge. Will He who did not condemn the ungodly man condemn the faithful man?
And yet, if you rightly consider, it was He who first gave you faith, whereby you obtained favor; for not of your own did you so obtain favor that anything should be due to you. Wherefore, then, in afterwards bestowing the reward of immortality, He crowns His own gifts, not your merits. Therefore, brethren, “we all of His fullness have received;” of the fullness of His mercy, of the abundance of His goodness have we received. What? The remission of sins that we might be justified by faith.
And what besides? “And grace for grace;” that is, for this grace by which we live by faith we shall receive another grace. What, then, is it except grace? For if I shall say that this also is due, I attribute something to myself as if to me it were due. But God crowns in us the gifts of His own mercy; but on condition that we walk with perseverance in that grace which in the first instance we received
Source: Tractates on the Gospel of John (New Advent)