Objection 1. It would seem that the debt of punishment arising through ingratitude in respect of a subsequent sin is as great as that of the sins previously pardoned. 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. But a slave who is freed by his master returns to the same state of slavery from which he was freed, or even to a worse state. Much more therefore he that sins against God after being freed from sin, returns to the debt of as great a punishment as he had incurred before.
Objection 3. Further, it is written (Matthew 18:34) that "his lord being angry, delivered him" (whose sins returned to him on account of his ingratitude) "to the torturers, until he paid all the debt." But this would not be so unless the debt of punishment incurred through ingratitude were as great as that incurred through all previous sins. Therefore an equal debt of punishment returns through ingratitude.
On the contrary, It is written (Deuteronomy 25:2): "According to the measure of the sin shall the measure also of the stripes be," whence it is evident that a great debt of punishment does not arise from a slight sin. But sometimes a subsequent mortal sin is much less grievous than any one of those previously pardoned. Therefore the debt of punishment incurred through subsequent sins is not equal to that of sins previously forgiven.
I answer that, Some have maintained that the debt of punishment incurred through ingratitude in respect of a subsequent sin is equal to that of the sins previously pardoned, in addition to the debt proper to this subsequent sin. But there is no need for this, because, as stated above (Article 1), the debt of punishment incurred by previous sins does not return on account of a subsequent sin, as resulting from the acts of the subsequent sin. Wherefore the amount of the debt that returns must be according to the gravity of the subsequent sin.
It is possible, however, for the gravity of the subsequent sin to equal the gravity of all previous sins. But it need not always be so, whether we speak of the gravity which a sin has from its species (since the subsequent sin may be one of simple fornication, while the previous sins were adulteries, murders, or sacrileges); or of the gravity which it incurs through the ingratitude connected with it. For it is not necessary that the measure of ingratitude should be exactly equal to the measure of the favor received, which latter is measured according to the greatness of the sins previously pardoned. Because it may happen that in respect of the same favor, one man is very ungrateful, either on account of the intensity of his scorn for the favor received, or on account of the gravity of the offense committed against the benefactor, while another man is slightly ungrateful, either because his scorn is less intense, or because his offense against the benefactor is less grave. But the measure of ingratitude is proportionately equal to the measure of the favor received: for supposing an equal contempt of the favor, or an equal offense against the benefactor, the ingratitude will be so much the greater, as the favor received is greater.
Hence it is evident that the debt of punishment incurred by a subsequent sin need not always be equal to that of previous sins; but it must be in proportion thereto, so that the more numerous or the greater the sins previously pardoned, the greater must be the debt of punishment incurred by any subsequent mortal sin whatever.
Reply to Objection 1. The favor of the pardon of sins takes its absolute quantity from the quantity of the sins previously pardoned: but the sin of ingratitude does not take its absolute quantity from the measure of the favor bestowed, but from the measure of the contempt or of the offense, as stated above: and so the objection does not prove.
Reply to Objection 2. A slave who has been given his freedom is not brought back to his previous state of slavery for any kind of ingratitude, but only when this is grave.
Reply to Objection 3. 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.
Source: Summa Theologica (New Advent)