J.I. Packer writes these sobering words of those who reject God:

Universalists suppose that the class of people mentioned in this heading will ultimately have no members, but the Bible indicates otherwise. Decisions made in this life will have eternal consequences. “Do not be deceived” (as you would be if you listened to the universalists), “God cannot be mocked. A man reaps what he sows” (Gal 6:7). Those who in this life reject God will forever be rejected by God. Universalism is the doctrine that, among others, Judas will be saved, but Jesus did not think he would. “The Son of Man will go just as it is written about him. But woe to that man who betrays the Son of Man! It would be better for him if he had not been born” (Mk 14:21) How could Jesus have spoken those last words if he had expected Judas finally to be saved?

Some, then face an eternity of rejectedness. How can we understand what they will bring on themselves? We cannot, of course, form an adequate notion of hell, any more than we can of heaven, and no doubt it is good for us that this is so; but perhaps the clearest notion we can form is that derived from contemplating the cross.

On the cross, God judged our sins in the person of his Son, and Jesus endured the retributive comeback of our wrongdoing. Look at the cross, therefore, and you see what form God’s judicial reaction to human sin will finally take. What form is that? In a word, withdrawal and deprivation of good. On the cross Jesus lost all the good that he had before: all sense of his Father’s presence and love, all sense of physical, mental and spiritual well-being, all enjoyment of God and of created things, all ease and solace of friendship, were taken from him, and in their place was nothing but loneliness, pain, a killing sense of human malice and callousness, and a horror of great spiritual darkness.

The physical pain, though great (for crucifixion remains the cruelest form of judicial execution that the world has ever known), was yet only a small part of the story; Jesus’ chief sufferings were mental and spiritual, and what was packed into less than four hundred minutes was an eternity of agony – agony such that each minute was an eternity in itself, as mental sufferers know that individual minutes can be.

So, too, those who reject God face the prospect of losing all good, and the best way to form an idea of eternal death is to dwell on this thought. In ordinary life, we never notice how much good we enjoy through God’s common grace till it is taken from us. We never value health, or steady circumstances, or friendship and respect from others, as we should till we have lost them. Calvary shows that under the final judgment of God nothing that one has valued, or could value, nothing that one can call good, remains to one. It is a terrible thought, but the reality, we may be sure, is more terrible yet. “It would be better for him if he had not been born.” God help us learn this lesson, which the spectacle of propitiation through penal substitution on the cross teaches so clearly; and may each of us be found in Christ, our sins covered by his blood, at the last.

(J.I. Packer, Knowing God, pp. 194-195)