Romans 4:5

Faith, which believes in the justifier, is the beginning of justification before God. And this faith, when it is justified, is like a root in the soil of the soul, which the rain has watered, so that as it begins to grow by the law of God, branches appear, which bring forth fruit. The root of righteousness does not spring from works; rather, the fruit of works grows from the root of righteousness, viz., by that root of righteousness by which God brings righteousness to the one whom he has accepted apart from works.

This refers to somebody who is bound by sin and who therefore does not do what the law commands. Paul says this because to an ungodly person, that is, to a Gentile, who believes in Christ without doing the works of the law, his faith is reckoned for righteousness just as Abraham’s was. How then can the Jews think that they have been justified by the works of the law in the same way as Abraham, when they see that Abraham was not justified by the works of the law but by faith alone? Therefore there is no need of the law when the ungodly is justified before God by faith alone.

For reflect how great a thing it is to be persuaded and have full confidence that God is able on a sudden not to free a man who has lived in impiety from punishment only, but even to make him just, and to count him worthy of those immortal honors. Do not then suppose that this one is lowered in that it is not reckoned unto the former of grace. For this is the very thing that makes the believer glorious; the fact of his enjoying so great grace, of his displaying so great faith. And note too that the recompense is greater. For to the former a reward is given, to the latter righteousness. Now righteousness is much greater than a reward. For righteousness is a recompense which most fully comprehends several rewards. Therefore after proving this from Abraham, he introduces David also as giving his suffrage in favor of the statement made. What then doth David say? and whom doth he pronounce blessed? is it him that triumphs in works, or him that hath enjoyed grace? him that hath obtained pardon and a gift? And when I speak of blessedness, I mean the chiefest of all good things; for as righteousness is greater than a reward, so is blessedness greater than righteousness.

When an ungodly person is converted, God justifies him by faith alone, not for good works which he does not have. On that basis he would have been punished for his ungodly works. But note that Paul does not say one who remains in sin is justified by faith but rather the ungodly, i.e., one who has just come to believe.

God makes the ungodly man godly, in order that he might persevere in this godliness and righteousness. For a man is justified in order that he might be just, not so that he might think it is all right to go on sinning.

The life of the soul is changeable. And when the soul lives in unrighteousness, that is its death; when again it becomes righteous, it becomes partaker of another life, which is not what itself is, inasmuch as by rising up to God, and cleaving to God, of Him it is justified. For it is said, "To him that believeth on Him that justifieth the ungodly, his faith is counted for righteousness." By forsaking God, it becomes unrighteous; by coming to Him, it is made righteous.

The one who believes, although he does not work, nevertheless presents on his part faith — a thing of great significance; for to be convinced that God can not only free from punishment one who has lived in ungodliness, but also make him righteous, is a matter of high value. For this reason, "to the one who believes... his faith is counted as righteousness," that is, God accepts his faith — not in order to give him wages, but in order to justify him. Therefore the one who believes brings something on his part as well, namely faith.

Then when he says but to him, he shows how the eternal reward is related to faith, saying, but to him who works not outward works, for example, because he does not have time to work, as in the case of one who dies immediately after baptism, yet believes in him who justifies the ungodly, namely, in God, of whom he says below: God is he who justifies (Rom 8:33), his faith is reputed, i.e., faith alone without outward works, to justice, so that in virtue of it he is called just and receives the reward of justice, just as if he had done the works of justice, as he says below: with the heart, we believe unto justice (Rom 10:10), and this according to the purpose of the grace of God, i.e., accordingly as God proposes to save men gratuitously: to such as, according to his purpose, are called to be saints (Rom 8:28); he accomplishes all things according to the counsel of his will (Eph 1:11).

Another explanation refers those words to man's justification.

He says therefore, to him who works, i.e., if anyone be justified by works, the justice would be reckoned as a reward, not according to grace, but according to debt: and if by grace, it is not now by works; otherwise grace is no more grace (Rom 11:6). But to him who works not, so as to be justified by his works, yet believes in him who justifies the ungodly, his faith is reputed to justice, according to the purpose of the grace of God, not that he merits justice through faith, but because the believing itself is the first act of the justice God works in him. For from the fact that he believes in God justifying, he submits himself to his justification and thus receives its effect.

This is the literal explanation and accords with the intention of the Apostle, who lays special stress on the words, it was reputed to him as justice (Gen 15:6), a saying which is used when that which is lacking on someone's part is reckoned to him gratis, as if he had accomplished the whole.

That is why the Apostle says that such reckoning would have no place, if justice were from works, but only as it is from faith.