Thomas Aquinas, Compendium of Theology

The suffering of eternal punishment is in no way opposed to divine justice. Even in the laws men make, punishment need not correspond to the offense in point of time. For the crime of adultery or murder, either of which may be committed in a brief span of time, human law may prescribe lifelong exile or even death, by both of which the criminal is banned forever from the society of the state. Exile, it is true, does not last forever, but this is purely accidental, owing to the fact that man's life is not everlasting; but the intention of the judge, we may assume, is to sentence the criminal to perpetual punishment, so far as he can. In the same way it is not unjust for God to inflict eternal punishment for a sin committed in a moment of time.

We should also take into consideration the fact that eternal punishment is inflicted on a sinner who does not repent of his sin, and so he continues in his sin up to his death. And since he is in sin for eternity, he is reasonably punished by God for all eternity. Furthermore, any sin committed against God has a certain infinity when regarded from the side of God, against whom it is committed. For, clearly, the greater the person who is offended, the more grievous is the offense. He who strikes a soldier is held more gravely accountable than if he struck a peasant; and his offense is much more serious if he strikes a prince or a king. Accordingly, since God is infinitely great, an offense committed against Him is in a certain respect infinite; and so a punishment that is in a certain respect infinite is duly attached to it. Such a punishment cannot be infinite in intensity, for nothing created can be infinite in this way. Consequently a punishment that is infinite in duration is rightly inflicted for mortal sin.

Moreover, while a person is still capable of correction, temporal punishment is imposed for his emendation or cleansing. But if a sinner is incorrigible, so that his will is obstinately fixed in sin, as we said above is the case with the damned, his punishment ought never to come to an end.