Objection 1. It would seem that every sin incurs a debt of eternal punishment. Because punishment, as stated above (Article 4), is proportionate to the fault. Now eternal punishment differs infinitely from temporal punishment: whereas no sin, apparently, differs infinitely from another, since every sin is a human act, which cannot be infinite. Since therefore some sins incur a debt of everlasting punishment, as stated above (Article 4), it seems that no sin incurs a debt of mere temporal punishment.
Objection 2. Further, original sin is the least of all sins, wherefore Augustine says (Enchiridion xciii) that "the lightest punishment is incurred by those who are punished for original sin alone." But original sin incurs everlasting punishment, since children who have died in original sin through not being baptized, will never see the kingdom of God, as shown by our Lord's words (John 3:3): " Unless a man be born again, he cannot see the kingdom of God." Much more, therefore, will the punishments of all other sins be everlasting.
Objection 3. Further, a sin does not deserve greater punishment through being united to another sin; for Divine justice has allotted its punishment to each sin. Now a venial sin deserves eternal punishment if it be united to a mortal sin in a lost soul, because in hell there is no remission of sins. Therefore venial sin by itself deserves eternal punishment. Therefore temporal punishment is not due for any sin.
On the contrary, Gregory says (Dial. iv, 39), that certain slighter sins are remitted after this life. Therefore all sins are not punished eternally.
I answer that, As stated above (Article 3), a sin incurs a debt of eternal punishment, in so far as it causes an irreparable disorder in the order of Divine justice, through being contrary to the very principle of that order, viz. the last end. Now it is evident that in some sins there is disorder indeed, but such as not to involve contrariety in respect of the last end, but only in respect of things referable to the end, in so far as one is too much or too little intent on them without prejudicing the order to the last end: as, for instance, when a man is too fond of some temporal thing, yet would not offend God for its sake, by breaking one of His commandments. Consequently such sins do not incur everlasting, but only temporal punishment.
Reply to Objection 1. Sins do not differ infinitely from one another in respect of their turning towards mutable good, which constitutes the substance of the sinful act; but they do differ infinitely in respect of their turning away from something. Because some sins consist in turning away from the last end, and some in a disorder affecting things referable to the end: and the last end differs infinitely from the things that are referred to it.
Reply to Objection 2. Original sin incurs everlasting punishment, not on account of its gravity, but by reason of the condition of the subject, viz. a human being deprived of grace, without which there is no remission of sin.
The same answer applies to the Third Objection about venial sin. Because eternity of punishment does not correspond to the quantity of the sin, but to its irremissibility, as stated above (Article 3).
Source: Summa Theologica (New Advent)