The Enchiridion by St. Augustine
Chapter 112.--There is No Ground in Scripture for the Opinion of Those Who Deny the Eternity of Future Punishments.
It is in vain, then, that some, indeed very many, make moan over the eternal punishment, and perpetual, unintermitted torments of the lost, and say they do not believe it shall be so; not, indeed, that they directly oppose themselves to Holy Scripture, but, at the suggestion of their own feelings, they soften down everything that seems hard, and give a milder turn to statements which they think are rather designed to terrify than to be received as literally true. For |Hath God| they say, forgotten to be gracious? hath He in anger shut up His tender mercies?| Now, they read this in one of the holy psalms. But without doubt we are to understand it as spoken of those who are elsewhere called |vessels of mercy,| because even they are freed from misery not on account of any merit of their own, but solely through the pity of God. Or, if the men we speak of insist that this passage applies to all mankind, there is no reason why they should therefore suppose that there will be an end to the punishment of those of whom it is said, |These shall go away into everlasting punishment;| for this shall end in the same manner and at the same time as the happiness of those of whom it is said, |but the righteous unto life eternal.| But let them suppose, if the thought gives them pleasure, that the pains of the damned are, at certain intervals, in some degree assuaged. For even in this case the wrath of God, that is, their condemnation (for it is this, and not any disturbed feeling in the mind of God that is called His wrath), abideth upon them; that is, His wrath, though it still remains, does not shut up His tender mercies; though His tender mercies are exhibited, not in putting an end to their eternal punishment, but in mitigating, or in granting them a respite from, their torments; for the psalm does not say, |to put an end to His anger,| or, |when His anger is passed by,| but |in His anger.| Now, if this anger stood alone, or if it existed in the smallest conceivable degree, yet to be lost out of the kingdom of God, to be an exile from the city of God, to be alienated from the life of God, to have no share in that great goodness which God hath laid up for them that fear Him, and hath wrought out for them that trust in Him, would be a punishment so great, that, supposing it to be eternal, no torments that we know of, continued through as many ages as man's imagination can conceive, could be compared with it.