Thomas Aquinas

“ If, however, a man, while intending to commit a sin, e.g. murder or adultery, is not withheld from it on account of its implying contempt of God, his ingratitude will not be a special sin, but will be drawn to the species of the other sin, as a circumstance thereof. ”

“ Because the greatness of the favor of the pardon of sins is according to the greatness of the sin pardoned, and so too, in consequence, is the greatness of the ingratitude whereby this favor is scorned. But the greatness of the consequent debt of punishment is in accord with the greatness of the ingratitude. Therefore the debt of punishment arising through ingratitude in respect of a subsequent sin is as great as the debt of punishment due for all the previous sins.
Objection 2: Further, it is a greater sin to offend God than to offend man. ”

“ Even venial sins and original sin return in the way explained above, just as mortal sins do, in so far as the favor conferred by God in forgiving those sins is despised. A man does not, however, incur ingratitude by committing a venial sin, because by sinning venially man does not act against God, but apart from Him, wherefore venial sins nowise cause the return of sins already forgiven. ”

“ Wherefore others who maintained the possibility of sins returning, said that God pardons the sins of a penitent who will afterwards sin again, not according to His foreknowledge, but only according to His present justice: since He foresees that He will punish such a man eternally for his sins, and yet, by His grace, He makes him righteous for the present. ”

“ If, however, this form be taken in regard to the penitent who receives this favor, we find on his part a twofold movement of the free-will. The first is the movement of the free-will towards God, and is an act of faith quickened by charity; and against this a man acts by apostatizing from the faith. The second is a movement of the free-will against sin, and is the act of penance. This act consists first, as we have stated above (Q [85] , AA [2] ,5) in man's detestation of his past sins; and against this a man acts when he regrets having done penance. ”

“ Hence it is evident, that on the part of the adherence, a subsequent mortal sin does not cause the return of mortal sins previously dispelled, else it would follow that by a sin of wastefulness a man would be brought back to the habit or disposition of avarice previously dispelled, so that one contrary would be the cause of another, which is impossible. ”

“ I answer that, As stated above ( [4792] Q [86] , A [4] ) , mortal sin contains two things, aversion from God and adherence to a created good. Now, in mortal sin, whatever attaches to the aversion, is, considered in itself, common to all mortal sins, since man turns away from God by every mortal sin, so that, in consequence, the stain resulting from the privation of grace, and the debt of everlasting punishment are common to all mortal sins. ”

“ But this is impossible, because what God has done cannot be undone by the work of man. Now the pardon of the previous sins was a work of Divine mercy, so that it cannot be undone by man's subsequent sin, according to Rom. ”

“ Yet it may happen that a subsequent sinful act virtually contains the debt of punishment due to the previous sin, in so far as when a man sins a second time, for this very reason he seems to sin more grievously than before, as stated in Rom. ”

“ This verse should make us tremble, we should not endeavor to explain it away lest through carelessness we give place to the sin which we thought to have been taken away, and become its slave once more. ”

“ He whose forgiven sins return to him on account of subsequent ingratitude, incurs the debt for all, in so far as the measure of his previous sins is contained proportionally in his subsequent ingratitude, but not absolutely, as stated above. ”

“ Dial. iv] ) : "It is evident from the words of the Gospel that if we do not forgive from our hearts the offenses committed against us, we become once more accountable for what we rejoiced in as forgiven through Penance": so that ingratitude implied in the hatred of one's brother is a special cause of the return of sins already forgiven: and the same seems to apply to the others.
I answer that, As stated above [4793] (A [1] ) , sins pardoned through Penance are said to return, in so far as their debt of punishment, by reason of ingratitude, is virtually contained in the subsequent sin. ”