Objection 1. It would seem that souls which depart with none but original sin, suffer from a bodily fire and are punished by fire. For Augustine [Fulgentius, De Fide ad Petrum, xxvii] says: "Hold firmly and doubt not that children who depart this life without the sacrament of Baptism will be punished everlastingly." Now punishment denotes sensible pain. Therefore souls which depart this life with original sin alone, suffer from a bodily fire and are tormented with the pain of fire.
Objection 2. Further, a greater fault deserves a greater punishment. Now original sin is greater than venial, because it contains more aversion, since it deprives its subject of grace, whereas venial sin is compatible with grace; and again because original sin is punished eternally, whereas venial sin is punished temporally. Seeing then that venial sin is deserving of the punishment of fire, much more so is original sin.
Objection 3. Further, sins are more severely punished after this life than during lifetime, for in this life there is room for mercy. Now, sensible punishment corresponds to original sin in this life, for children who have only original sin are justly subject to many sensible punishments. Therefore sensible punishment is due to it after this life.
Objection 4. Further, even as in actual sin there is aversion and conversion, so in original sin there is something corresponding to aversion, namely the privation of original justice, and something corresponding to conversion, namely concupiscence. Now the punishment of fire is due to actual sin by reason of the conversion. Therefore it is also due to original sin by reason of concupiscence.
Objection 5. Further, after the resurrection the bodies of children will be either passible or impassible. If they be impassible--and no human body can be impassible except either on account of the gift of impassibility (as in the blessed) or by reason of original justice (as in the state of innocence)--it follows that the bodies of children will either have the gift of impassibility, and thus will be glorious, so that there will be no difference between baptized and non-baptizedchildren, which is heretical, or else they will have original justice, and thus will be without original sin, and will not be punished for original sin, which is likewise heretical. If, on the other hand, they be passible, since everything passible suffers of necessity in the presence of the active, it follows that in the presence of active sensible bodies they will suffer sensible punishment.
On the contrary, Augustine says (Enchiridion xxiii) that the mildest punishment of all will be for those who are burdened with original sin only. But this would not be so, if they were tormented with sensible punishment, because the pain of hell fire is most grievous. Therefore they will not suffer sensible punishment.
Further, the grief of sensible punishment corresponds to the pleasure of sin (Apocalypse 18:7): "As much as she hath glorified herself and lived in delicacies, so much torment and sorrow give ye to her." But there is no pleasure in original sin, as neither is there operation, for pleasure follows operation, as stated in Ethic. x, 4. Therefore punishment by fire is not due to original sin.
Further, Gregory Nazianzen in his fortieth sermon, which is entitled On Holy Baptism, distinguishes three classes of unbaptized persons: those namely who refuse to be baptized, those who through neglect have put off being baptized until the end of life and have been surprised by sudden death, and those who, like infants, have failed to receive it through no fault of theirs. Of the first he says that they will be punished not only for their other sins, but also for their contempt of Baptism; of the second, that they will be punished, though less severely than the first, for having neglected it; and of the last he says that "a just and eternal Judge will consign them neither to heavenly glory nor to the eternal pains of hell, for although they have not been signed with Baptism, they are without wickedness and malice, and have suffered rather than caused their loss of Baptism." He also gives the reason why, although they do not reach the glory of heaven, they do not therefore suffer the eternal punishment suffered by the damned: "Because there is a mean between the two, since he who deserves not honor and glory is not for that reason worthy of punishment, and on the other hand he who is not deserving of punishment is not for that reason worthy of glory and honor."
I answer that, Punishment should be proportionate to fault, according to the saying of Isaias (27:8), "In measure against measure, when it shall be cast off, thou shalt judge it." Now the defect transmitted to us through our origin, and having the character of a sin does not result from the withdrawal or corruption of a good consequent upon human nature by virtue of its principles, but from the withdrawal or corruption of something that had been superadded to nature. Nor does this sin belong to this particular man, except in so far as he has such a nature, that is deprived of this good, which in the ordinary course of things he would have had and would have been able to keep. Wherefore no further punishment is due to him, besides the privation of that end to which the gift withdrawn destined him, which gift human nature is unable of itself to obtain. Now this is the divine vision; and consequently the loss of this vision is the proper and only punishment of original sin after death: because, if any other sensible punishment were inflicted after death for original sin, a man would be punished out of proportion to his guilt, for sensible punishment is inflicted for that which is proper to the person, since a man undergoes sensible punishment in so far as he suffers in his person. Hence, as his guilt did not result from an action of his own, even so neither should he be punished by suffering himself, but only by losing that which his nature was unable to obtain. On the other hand, those who are under sentence for original sin will suffer no loss whatever in other kinds of perfection and goodness which are consequent upon human nature by virtue of its principles.
Reply to Objection 1. In the authority quoted punishment denotes, not pain of sense, but only pain of loss, which is the privation of the divine vision, even as in Scripture the word "fire" is often wont to signify any kind of punishment.
Reply to Objection 2. Of all sins original sin is the least, because it is the least voluntary; for it is voluntary not by the will of the person, but only by the will of the origin of our nature. But actual sin, even venial, is voluntary by the will of the person in which it is; wherefore a lighter punishment is due to original than to venial sin. Nor does it matter that original sin is incompatible with grace; because privation of grace has the character, not of sin, but of punishment, except in so far as it is voluntary: for which reason that which is less voluntary is less sinful. Again it matters not that actual venial sin is deserving of temporal punishment, since this is accidental, for as much as he who falls venially has sufficient grace to attenuate the punishment. For if venial sin were in a person without grace, it would be punished eternally.
Reply to Objection 3. There is no parity between pain of sense before and after death, since before death the pain of sense results from the power of the natural agent, whether the pain of sense be interior as fever or the like, or exterior as burning and so forth. Whereas after death nothing will act by natural power, but only according to the order of divine justice, whether the object of such action be the separate soul, on which it is clear that fire cannot act naturally, or the body after resurrection, since then all natural action will cease, through the cessation of the first movable which is the cause of all bodily movement and alteration.
Reply to Objection 4. Sensible pain corresponds to sensible pleasure, which is in the conversion of actual sin: whereas habitual concupiscence, which is in original sin, has no pleasure. Hence, sensible pain does not correspond thereto as punishment.
Reply to Objection 5. The bodies of children will be impassible, not through their being unable in themselves to suffer, but through the lack of an external agent to act upon them: because, after the resurrection, no body will act on another, least of all so as to induce corruption by the action of nature, but there will only be action to the effect of punishing them by order of the divine justice. Wherefore those bodies to which pain of sense is not due by divine justice will not suffer punishment. On the other hand, the bodies of the saints will be impassible, because they will lack the capability of suffering; hence impassibility in them will be a gift, but not in children.
Source: Summa Theologica (New Advent)