When then man by grace has obtained pardon for his sin and has been restored to the state of grace, he still remains bound by God's justice to some punishment for his sin. If of his own will he exacts this punishment of himself, he is thereby said to 'make satisfaction' to God, inasmuch as by punishing himself for his sin he fulfils with labour and pain the order instituted of heaven, which order he had transgressed by sinning and following his own will. But if he does not exact this punishment of himself it will be inflicted by God, since the domain of divine providence cannot be suffered to lie in disorder. The punishment in that case will not be called 'satisfactory,' since it will not be of the choice of the sufferer, but it will be called 'purificatory,' or 'purgatorial,' because he will be purified and purged by another punishing him; and so whatever was inordinate in him will be brought back to due order. Hence the Apostle says: If we were to judge ourselves, we should not be judged: but while we are judged, we are chastised by the Lord, that we may not be condemned with this world (1 Cor. xi, 31).
Nevertheless, in the process of conversion, the disgust for sin and the fixing of the affections on God may be so intense as that there shall remain no outstanding liability to punishment. For the punishment which one suffers after the forgiveness of sin is necessary to bring the mind to cleave more firmly to good, -- punishments being medicines, -- as also for the observance of the order of justice in the punishment of the sinner. But love of God, especially when it is vehement and strong, is sufficient to establish man's mind in good; and intense disgust for a past fault carries with it great sorrow for the same. Hence by the vehemence of the love of God and hatred for sin there is excluded any further need of satisfactory or purgatorial punishment.* And though the vehemence be not so great as totally to bar the punishment, nevertheless, the greater the vehemence, so much less of punishment will suffice.
But what we do through our friends we are reckoned to do of ourselves, inasmuch as friendship makes two one in heart, and this is especially true of the love of charity: therefore, as a man may make satisfaction to God of himself, so also may he do it through another, especially in case of necessity: for the punishment which his friend suffers on his account he reckons as his own punishment; and thus punishment is not wanting to him in that he has compassion for the sufferings of his friend, and that all the more for his being the cause of his friend's suffering. And again the affection of charity in him who suffers for his friend makes his satisfaction more acceptable to God than it would be if he were suffering for his own doings: for the one is an effort of spontaneous charity, the other an acquiescence in necessity. Hence we learn that one man may make satisfaction for another, provided both of them be in charity. Hence the saying of the Apostle: Bear ye one another's burdens, and so ye shall fulfil the law of Christ (Gal. vi, 2).
3.158 : That Man cannot be delivered from Sin except by Grace
3.160 : That it is reasonably reckoned a Man's own Fault if he be not converted to God, although he cannot be converted without Grace