Objection 1. It would seem that man cannot make satisfaction to God. For satisfaction should balance the offense, as shown above (12, 2,3). But an offense against God is infinite, since it is measured by the person against whom it is committed, for it is a greater offense to strike a prince than anyone else. Therefore, as no action of man can be infinite, it seems that he cannot make satisfaction to God.
Objection 2. Further, a slave cannot make compensation for a debt, since all that he has is his master's. But we are the slaves of God, and whatever good we have, we owe to Him. Therefore, as satisfaction is compensation for a past offense, it seems that we cannot offer it to God.
Objection 3. Further, if all that a man has suffices not to pay one debt, he cannot pay another debt. Now all that man is, all that he can do, and all that he has, does not suffice to pay what he owes for the blessing of creation, wherefore it is written (Isaiah 40:16) that "the wood of Libanus shall not be enough for a burnt offering [Vulgate: 'Libanus shall not be enough to burn, nor the beasts thereof for a burnt offering']." Therefore by no means can he make satisfaction for the debt resulting from the offense committed.
Objection 4. Further, man is bound to spend all his time in the service of God. Now time once lost cannot be recovered, wherefore, as Seneca observes (Lib. i, Ep. i, ad Lucilium) loss of time is a very grievous matter. Therefore man cannot make compensation to God, and the same conclusion follows as before.
Objection 5. Further, mortal actual sin is more grievous than original sin. But none could satisfy for original sin unless he were both God and man. Neither, therefore, can he satisfy for actual sin.
On the contrary, Jerome [Pelagius, Expos. Fidei ad Damasum] says: "Whoever maintains that God has commanded anything impossible to man, let him be anathema." But satisfaction is commanded (Luke 3:8): "Bring forth... fruits worthy of penance." Therefore it is possible to make satisfaction to God.
Further, God is more merciful than any man. But it is possible to make satisfaction to a man. Therefore it is possible to make satisfaction to God.
Further, there is due satisfaction when the punishment balances the fault, since "justice is the same as counterpassion," as the Pythagoreans said [Aristotle, Ethic. v, 5; Cf. II-II, 61, 4]. Now punishment may equal the pleasure contained in a sin committed. Therefore satisfaction can be made to God.
I answer that, Man becomes God's debtor in two ways; first, by reason of favors received, secondly, by reason of sin committed: and just as thanksgiving or worship or the like regard the debt for favors received, so satisfaction regards the debt for sin committed. Now in giving honor to one's parents or to the gods, as indeed the Philosopher says (Ethic. viii, 14), it is impossible to repay them measure for measure, but it suffices that man repay as much as he can, for friendship does not demand measure for measure, but what is possible. Yet even this is equal somewhat, viz. according to proportion, for as the debt due to God is, in comparison with God, so is what man can do, in comparison with himself, so that in another way the form of justice is preserved. It is the same as regards satisfaction. Consequently man cannot make satisfaction to God if "satis" [enough] denotes quantitative equality; but he can, if it denote proportionate equality, as explained above, and as this suffices for justice, so does it suffice for satisfaction.
Reply to Objection 1. Just as the offense derived a certain infinity from the infinity of the Divine majesty, so does satisfaction derive a certain infinity from the infinity of Divine mercy, in so far as it is quickened by grace, whereby whatever man is able to repay becomes acceptable. Others, however, say that the offense is infinite as regards the aversion, and in this respect it is pardoned gratuitously, but that it is finite as turning to a mutable good, in which respect it is possible to make satisfaction for it. But this is not to the point, since satisfaction does not answer to sin, except as this is an offense against God, which is a matter, not of turning to a creature but of turning away from God. Others again say that even as regards the aversion it is possible to make satisfaction for sin in virtue of Christ's merit, which was, in a way, infinite. And this comes to the same as what we said before, since grace is given to believers through faith in the Mediator. If, however, He were to give grace otherwise, satisfaction would suffice in the way explained above.
Reply to Objection 2. Man, who was made to God's image, has a certain share of liberty, in so far as he is master of his actions through his free-will; so that, through acting by his free-will, he can make satisfaction to God, for though it belongs to God, in so far as it was bestowed on him by God, yet it was freely bestowed on him, that he might be his own master, which cannot be said of a slave.
Reply to Objection 3. This argument proves that it is impossible to make equivalent satisfaction to God, but not that it is impossible to make sufficient satisfaction to Him. For though man owes God all that he is able to give Him, yet it is not necessary for his salvation that he should actually do the whole of what he is able to do, for it is impossible for him, according to his present state of life, to put forth his whole power into any one single thing, since he has to be heedful about many things. And so his conduct is subject to a certain measure, viz. the fulfillment of God's commandments, over and above which he can offer something by way of satisfaction.
Reply to Objection 4. Though man cannot recover the time that is past, he can in the time that follows make compensation for what he should have done in the past, since the commandment did not exact from him the fulfillment of his whole power, as stated above (ad 3).
Reply to Objection 5. Though original sin has less of the nature of sin than actual sin has, yet it is a more grievous evil, because it is an infection of human nature itself, so that, unlike actual sin, it could not be expiated by the satisfaction of a mere man.
Source: Summa Theologica (New Advent)