Objection 1. It would seem that no sin incurs a debt of eternal punishment. For a just punishment is equal to the fault, since justice is equality: wherefore it is written (Isaiah 27:8): "In measure against measure, when it shall be cast off, thou shalt judge it." Now sin is temporal. Therefore it does not incur a debt of eternal punishment.
Objection 2. Further, "punishments are a kind of medicine" (Ethic. ii, 3). But no medicine should be infinite, because it is directed to an end, and "what is directed to an end, is not infinite," as the Philosopher states (Polit. i, 6). Therefore no punishment should be infinite.
Objection 3. Further, no one does a thing always unless he delights in it for its own sake. But "God hath not pleasure in the destruction of men" [Vulgate: 'of the living']. Therefore He will not inflict eternal punishment on man.
Objection 4. Further, nothing accidental is infinite. But punishment is accidental, for it is not natural to the one who is punished. Therefore it cannot be of infinite duration.
On the contrary, It is written (Matthew 25:46): "These shall go into everlasting punishment"; and (Mark 3:29): "He that shall blaspheme against the Holy Ghost, shall never have forgiveness, but shall be guilty of an everlasting sin."
I answer that, As stated above (Article 1), sin incurs a debt of punishment through disturbing an order. But the effect remains so long as the cause remains. Wherefore so long as the disturbance of the order remains the debt of punishment must needs remain also. Now disturbance of an order is sometimes reparable, sometimes irreparable: because a defect which destroys the principle is irreparable, whereas if the principle be saved, defects can be repaired by virtue of that principle. For instance, if the principle of sight be destroyed, sight cannot be restored except by Divine power; whereas, if the principle of sight be preserved, while there arise certain impediments to the use of sight, these can be remedied by nature or by art. Now in every order there is a principle whereby one takes part in that order. Consequently if a sin destroys the principle of the order whereby man's will is subject to God, the disorder will be such as to be considered in itself, irreparable, although it is possible to repair it by the power of God. Now the principle of this order is the last end, to which man adheres by charity. Therefore whatever sins turn man away from God, so as to destroy charity, considered in themselves, incur a debt of eternal punishment.
Reply to Objection 1. Punishment is proportionate to sin in point of severity, both in Divine and in human judgments. In no judgment, however, as Augustine says (De Civ. Dei xxi, 11) is it requisite for punishment to equal fault in point of duration. For the fact that adultery or murder is committed in a moment does not call for a momentary punishment: in fact they are punished sometimes by imprisonment or banishment for life--sometimes even by death; wherein account is not taken of the time occupied in killing, but rather of the expediency of removing the murderer from the fellowship of the living, so that this punishment, in its own way, represents the eternity of punishment inflicted by God. Now according to Gregory (Dial. iv, 44) it is just that he who has sinned against God in his own eternity should be punished in God's eternity. A man is said to have sinned in his own eternity, not only as regards continual sinning throughout his whole life, but also because, from the very fact that he fixes his end in sin, he has the will to sin, everlastingly. Wherefore Gregory says (Dial. iv, 44) that the "wicked would wish to live without end, that they might abide in their sins for ever."
Reply to Objection 2. Even the punishment that is inflicted according to human laws, is not always intended as a medicine for the one who is punished, but sometimes only for others: thus when a thief is hanged, this is not for his own amendment, but for the sake of others, that at least they may be deterred from crime through fear of the punishment, according to Proverbs 19:25: "The wicked man being scourged, the fool shall be wiser." Accordingly the eternal punishments inflicted by God on the reprobate, are medicinal punishments for those who refrain from sin through the thought of those punishments, according to Psalm 59:6: "Thou hast given a warning to them that fear Thee, that they may flee from before the bow, that Thy beloved may be delivered."
Reply to Objection 3. God does not delight in punishments for their own sake; but He does delight in the order of His justice, which requires them.
Reply to Objection 4. Although punishment is related indirectly to nature, nevertheless it is essentially related to the disturbance of the order, and to God's justice. Wherefore, so long as the disturbance lasts, the punishment endures.
Source: Summa Theologica (New Advent)