1. Satisfaction may be said to be sufficient in two ways – first, perfectly, inasmuch as it is condign, being adequate to make good the fault committed, and in this way the satisfaction of a mere man cannot be sufficient for sin, both because the whole of human nature had been corrupted by sin, whereas the goodness of any person or persons could not make up adequately for the injury done to the whole of human nature, and also because a sin committed against God has a kind of infinity from the infinity of the Divine majesty, because the greater the person we offend, the more grievous the offense. Hence for condign satisfaction, it was necessary that the act of the one satisfying should have an infinite efficacy, as being of God and man.

Secondly, man’s satisfaction may be called sufficient imperfectly, that is , in the acceptation of him who is content with it, even though it is not condign and in this way the satisfaction of a mere man is sufficient. And inasmuch as everything imperfect presupposes some perfect thing by which it is sustained, hence it is that the satisfaction of every mere man has its efficiency from the satisfaction of Christ.

2. The Incarnation gives us confidence in regard to the forgiveness of sins.

Just as virtue prepares man for heaven, so sin debars him therefrom. Now sin, which is opposed to virtue, debars man from heaven not only because it brings disorder into the soul by leading it away from its proper end, but also because it offends God, to Whom, as the Director of human actions, man looks for his heavenly reward. Moreover, when a man is conscious of sin he loses hope, which he needs in order to reach heaven. Therefore, as sin abounds in the human race man needs a remedy for it.

But no one can provide this remedy except God alone, Who is able not only to move man’s will to good, so as to bring him back to the right order, but also to forgive the offence committed against Himself; since an offence is not forgiven except by the person offended.

But in order that man’s conscience may be eased of his past sin he must be made certain of God’s forgiveness. But he cannot be certain of this except by God himself. Hence it was fitting to the human race and expedient for the possession of heavenly bliss, that God should become man, so that man would both receive from God the forgiveness of his sins and be made certain of that forgiveness by God made man. Hence Our Lord said (Matt. 9:6): But that you may know that the Son of man has authority on earth to forgive sins”; and the Apostle says, (Heb. 9:14): the blood of Christ, who through the eternal Spirit offered himself without blemish to God, purify your conscience from dead works to serve the living God.” (Contra Gentiles IV, 54).