The human hell is weeping (inconceivable pain) and gnashing of teeth (utter hatred) mixed into a package of eternal horror. It is impossible, let it be proposed, that a perfectly good God would let any of His creatures suffer like that, including even the demons.
This reasoning runs into two problems. (1) Why does Jesus warn us on many occasions of hell? (2) Aren’t demons already condemned to hell?
(1) I think, though cannot prove definitively, that the human hell exists not as an ultimate place or state of punishment but as an ultimate incentive against moral evil. It is meant to be avoided by all, just as it is best that a human law threatening punishment for stealing is never violated. In other words, if any man, no matter how brutal or sinful or selfish or wild, were to feel hellfire, then he would become so afraid that he’d immediately embark upon a long path of self-reformation. It is still possible go to hell; but in fact the human hell is actually empty, as no one fails to heed its horror if exposed to it.
Of course, hellfire is God’s final tool when everything else has failed. Very few people are given this grace.
(2) Demons are condemned, but they go to their own demonic hell which is different from human hell. It does not feature pain by hellfire. Instead, the demons will experience in their hell
- separation from God and impossibility to interact with Him naturally,
- privation of any grace and knowledge of divine secrets in the state of glory,
- environment less pleasant than either their heavenly home from which they fell or this universe into which they were banished and where they now reside,
- sorrow from inability to satisfy their desire of harming humans,
- sorrow from being completely defeated by creatures they used to despise — us.
If the wicked angels believed they would, upon defeat, go to anything like the human hell, they would never have rebelled. Satisfaction of pride, no matter how burning, in refusing to serve humans, is a limited finite end; eternal torture is an infinite evil. Surely, every angel was smart enough to weigh the probabilities and agree to receive the Holy Spirit’s grace, if the alternative was a chance of being utterly destroyed and tormented forever.
Thus, the problem of infinite horror is neatly solved. The human hell is absolute evil but is supposed to and does stay empty (though avoiding it is each person’s task; there is no limit to how much one can sin and suffer before he comes to his senses), functioning merely as a perfect deterrent; the demonic hell will be full of filth, but the prisoners there will simply be isolated and forgotten and miserable from pain of loss or privation of the goods proper to angels both in nature and glory, but they will not suffer hellfire.
Note that even if these two objections to universal salvation are solved correctly, this is merely evidence in favor of the conclusion, not a proof. That it does not befit God to torture creatures for all eternity is merely my own intuition. Perhaps my heart is unbecomingly soft. Nevertheless, these ruminations may be of interest.