Objection 1. It would seem that sin incurs a debt of punishment infinite in quantity. For it is written (Jeremiah 10:24): "Correct me, O Lord, but yet with judgment: and not in Thy fury, lest Thou bring me to nothing." Now God's anger or fury signifies metaphorically the vengeance of Divine justice: and to be brought to nothing is an infinite punishment, even as to make a thing out of nothing denotes infinite power. Therefore according to God's vengeance, sin is awarded a punishment infinite in quantity.
Objection 2. Further, quantity of punishment corresponds to quantity of fault, according to Deuteronomy 25:2: "According to the measure of the sin shall the measure also of the stripes be." Now a sin which is committed against God, is infinite: because the gravity of a sin increases according to the greatness of the person sinned against (thus it is a more grievous sin to strike the sovereign than a private individual), and God's greatness is infinite. Therefore an infinite punishment is due for a sin committed against God.
Objection 3. Further, a thing may be infinite in two ways, in duration, and in quantity. Now the punishment is infinite in duration. Therefore it is infinite in quantity also.
On the contrary, If this were the case, the punishments of all mortal sins would be equal; because one infinite is not greater than another.
I answer that, Punishment is proportionate to sin. Now sin comprises two things. First, there is the turning away from the immutable good, which is infinite, wherefore, in this respect, sin is infinite. Secondly, there is the inordinate turning to mutable good. In this respect sin is finite, both because the mutable good itself is finite, and because the movement of turning towards it is finite, since the acts of a creature cannot be infinite. Accordingly, in so far as sin consists in turning away from something, its corresponding punishment is the "pain of loss," which also is infinite, because it is the loss of the infinite good, i.e. God. But in so far as sin turns inordinately to something, its corresponding punishment is the "pain of sense," which is also finite.
Reply to Objection 1. It would be inconsistent with Divine justice for the sinner to be brought to nothing absolutely, because this would be incompatible with the perpetuity of punishment that Divine justice requires, as stated above (Article 3). The expression "to be brought to nothing" is applied to one who is deprived of spiritual goods, according to 1 Corinthians 13:2: "If I... have not charity, I am nothing."
Reply to Objection 2. This argument considers sin as turning away from something, for it is thus that man sins against God.
Reply to Objection 3. Duration of punishment corresponds to duration of fault, not indeed as regards the act, but on the part of the stain, for as long as this remains, the debt of punishment remains. But punishment corresponds to fault in the point of severity. And a fault which is irreparable, is such that, of itself, it lasts for ever; wherefore it incurs an everlasting punishment. But it is not infinite as regards the thing it turns to; wherefore, in this respect, it does not incur punishment of infinite quantity.
Source: Summa Theologica (New Advent)