City of God

But eternal punishment seems hard and unjust to human perceptions, because in the weakness of our mortal condition there is wanting that highest and purest wisdom by which it can be perceived how great a wickedness was committed in that first transgression. The more enjoyment man found in God, the greater was his wickedness in abandoning Him; and he who destroyed in himself a good which might have been eternal, became worthy of eternal evil. Hence the whole mass of the human race is condemned; for he who at first gave entrance to sin has been punished with all his posterity who were in him as in a root, so that no one is exempt from this just and due punishment, unless delivered by mercy and undeserved grace; and the human race is so apportioned that in some is displayed the efficacy of merciful grace, in the rest the efficacy of just retribution.

For both could not be displayed in all; for if all had remained under the punishment of just condemnation, there would have been seen in no one the mercy of redeeming grace. And, on the other hand, if all had been transferred from darkness to light, the severity of retribution would have been manifested in none. But many more are left under punishment than are delivered from it, in order that it may thus be shown what was due to all. And had it been inflicted on all, no one could justly have found fault with the justice of Him who takes vengeance; whereas, in the deliverance of so many from that just award, there is cause to render the most cordial thanks to the gratuitous bounty of Him who delivers.