Objection 1. It would seem that sins do not return through ingratitude, which is shown especially in four kinds of sin, viz., hatred of one's neighbor, apostasy from faith, contempt of confession and regret for past repentance, and which have been expressed in the following verse:
"Fratres odit, apostata fit, spernitque, fateri,
Poenituisse piget, pristina culpa redit."
sin
God
grace
sins
blasphemy
God
sin
Holy Ghost
sins
sins
sins
Objection 2. Further, Rabanus says: "God delivered the wicked servant to the torturers, until he should pay the whole debt, because a man will be deemed punishable not only for the sins he commits after Baptism, but also for original sin which was taken away when he was baptized." Now venial sins are reckoned among our debts, since we pray in their regard: "Forgive us our trespasses [debita]." Therefore they too return through ingratitude; and, in like manner seemingly, sins already pardoned return through venial sins, and not only through those sins mentioned above.
Objection 3. Further, ingratitude is all the greater, according as one sins after receiving a greater favor. Now innocence whereby one avoids sin is a Divine favor, for Augustine says (Confess. ii): "Whatever sins I have avoided committing, I owe it to Thy grace." Now innocence is a greater gift, than even the forgiveness of all sins. Therefore the first sin committed after innocence is no less an ingratitude to God, than a sin committed after repentance, so that seemingly ingratitude in respect of the aforesaid sins is not the chief cause of sins returning.
On the contrary, Gregory says (Moral. xviii [Cf. 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 (Article 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. Now one may be guilty of ingratitude in two ways: first by doing something against the favor received, and, in this way, man is ungrateful to God in every mortal sin whereby he offends God Who forgave his sins, so that by every subsequent mortal sin, the sins previously pardoned return, on account of the ingratitude. Secondly, one is guilty of ingratitude, by doing something not only against the favor itself, but also against the form of the favor received. If this form be considered on the part of the benefactor, it is the remission of something due to him; wherefore he who does not forgive his brother when he asks pardon, and persists in his hatred, acts against this form. 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 (85, 2,5) in man's detestation of his past sins; and against this a man acts when he regrets having done penance. Secondly, the act of penance consists in the penitent purposing to subject himself to the keys of the Church by confession, according to Psalm 31:5: "I said: I will confess against myself my injustice to the Lord: and Thou hast forgiven the wickedness of my sin": and against this a man acts when he scorns to confess as he had purposed to do.
Accordingly it is said that the ingratitude of sinners is a special cause of the return of sins previously forgiven.
Reply to Objection 1. This is not said of these sins as though they were more grievous than others, but because they are more directly opposed to the favor of the forgiveness of sin.
Reply to Objection 2. 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.
Reply to Objection 3. A favor can be weighed in two ways. First by the quantity of the favor itself, and in this way innocence is a greater favor from God than penance, which is called the second plank after shipwreck (cf. 84, 6). Secondly, a favor may be weighed with regard to the recipient, who is less worthy, wherefore a greater favor is bestowed on him, so that he is the more ungrateful if he scorns it. In this way the favor of the pardon of sins is greater when bestowed on one who is altogether unworthy, so that the ingratitude which follows is all the greater.
Source: Summa Theologica (New Advent)