9 What, then, is “grace for grace”? By faith we render God favorable to us; and inasmuch as we were not worthy to have our sins forgiven, and because we, who were unworthy, received so great a benefit, it is called grace. What is grace? That which is freely given. What is “freely given”? Given, not paid. If it was due, wages were given, not grace bestowed; but if it was reply due, you were good; but if, as is true, you were evil, but believed in Him who justifies the ungodly (What is, Who justifies the ungodly?
Of the ungodly makes pious), consider what did by right hang over you by the law, and what you have obtained by grace. But having obtained that grace of faith, you shall be just by faith (for the just lives by faith); and you shall obtain favor of God by living by faith. And having obtained favor from God by living by faith, you shall receive immortality as a reward, and life eternal. And that is grace. For because of what merit do you receive life eternal? Because of grace. For if faith is grace, life eternal is, as it were, the wages of faith: God, indeed, appears to bestow eternal life as if it were due (To whom due? To the faithful, because he had merited it by faith); but because faith itself is grace, life eternal also is grace for grace.
Source: Tractates on the Gospel of John (New Advent)