The Enchiridion by St. Augustine
Chapter 107.--Eternal Life, Though the Reward of Good Works, is Itself the Gift of God.
Wherefore, even eternal life itself, which is surely the reward of good works, the apostle calls the gift of God. |For the wages of sin,| he says, |is death; but the gift of God is eternal life through Jesus Christ our Lord.| Wages (stipendium) is paid as a recompense for military service; it is not a gift: wherefore he says, |the wages of sin is death,| to show that death was not inflicted undeservedly, but as the due recompense of sin. But a gift, unless it is wholly unearned, is not a gift at all. We are to understand, then, that man's good deserts are themselves the gift of God, so that when these obtain the recompense of eternal life, it is simply grace given for grace. Man, therefore, was thus made upright that, though unable to remain in his uprightness without divine help, he could of his own mere will depart from it. And whichever of these courses he had chosen, God's will would have been done, either by him, or concerning him. Therefore, as he chose to do his own will rather than God's, the will of God is fulfilled concerning him; for God, out of one and the same heap of perdition which constitutes the race of man, makes one vessel to honor, another to dishonor; to honor in mercy, to dishonor in judgment; that no one may glory in man, and consequently not in himself.