Hell is fundamentally an expression of God's love and mercy.
Most Christians would treat eternal damnation as a pretty clear instance of divine justice. God is perfect; we are not. If we are not redeemed by Christ's sacrifice, we are therefore outcasts from the Kingdom and doomed to eternal punishment.
This is all well and good, until we try to understand this doctrine in the context of a loving God. Questions abound: What sort of loving God would even allow for eternal punishment of His children? What sort of forgiving God would base such punishment on the choices we made based on incomplete knowledge in an imperfect world? Where is the justice in eternal consequences for choices made in a single lifetime? Wouldn't annihilation be more loving and more just than eternal damnation: finite suffering for finite sin?
I don't imagine I can satisfactorily answer these questions, but that is not my goal. I want to develop a positive, systematic doctrine of Hell, rather than a negative, piecemeal response to the doctrine's critics.
I can hardly keep myself from smiling when I consider some of the worship songs we sing regularly in the Christian community. Is it not laughably ironic that we sing about the "Refiner's Fire" in a major harmonic chord? I would imagine a rendition by Underoath or a screamo band would be more appropriate: something harsh, raspy, and unpleasant. After all, this is as close to a doctrine of Purgatory as most Protestants come: we are brought into the Presence of God and our sins and impurities are burnt out of us. Can we fathom the pain of this restoration? No, of course not, any more than we can fathom the joy and relief of coming out on the other side as new creatures who will judge angels and fully reflect the image of God.
Let us take this as our starting point: when we enter the Presence of God, we will be purified.
Some will object that this ignores the blood and sacrifice of Jesus Christ, a sacrifice that was sufficient to redeem us from sin, and restore us to a life with the Father through the Holy Spirit. Truly we have been made new creations. Yet it is no heresy to state that we are daily put to the test, that we walk through trial and tribulation, and that our faith is purified by fire and water. Therefore, I believe that the doctrine of Purgatory -- however we view it, whether in life or after death -- is no heresy, but falls squarely within our experience of our journey of faith. We will be purified.
Why do we endure these labor pangs of the afterlife? Because of our love for our Creator, because of the hope of salvation, because of the Joy of His Presence that is promised to us as children of God.
Yet Christ promised us that there will be some who reject Him, who are herded to His left with the goats. We must accept that some individuals are morally degenerate, others who are moral cowards, and some who are simply apathetic. There are heretics and nihilists, epicures and materialists, and a vast number of people who Just Don't Care. These individuals will not accept our reasons for accepting the pain of His Presence. They do not hold to the hope of salvation, and will not assent to the pain of purification. They do not participate in our love for God and each other, and will not pay the price of admission into His Presence. What happens to them?
Will Christ force them to endure the 'refining fire'? Will He drag them against their will, kicking and screaming, across the threshold of Paradise? Heaven forbid! My loving God is no tyrant; this bait-and-switch of salvation by faith supplanted by divine fiat is unworthy of a just God. True divine love dictates that, having given humans the dignity of free will, God allows them the freedom to exercise it and face the consequences of their decisions.
If we continue to defy God and resist submitting to Him with the words "Thy will be done," the day will come when God grants us our wish and says to us, "Thy will be done."
It is an expression of love that God grants us the dignity of creatures bearing His image. It is an expression of justice that God requires us to be purified before fully entering His presence. It is an expression of mercy that God allows us to choose an eternity of self-will outside of His presence.
For that is Hell: an existence outside of His Presence. God withdraws Himself sufficiently to allow these individuals respite from His Presence. It is a worse fate than we can imagine, for all the evil we can ever experience is tainted by the original goodness of Creation, and by the sustaining Presence of God.
It is also a mystery deeper than we can fathom, for God is the Substance and sustainer of reality, and nothing exists outside of Him. Evil is the negation of God, and has no substance; yet those damned (cast away from the Presence) continue to exist and enjoy the privileges of existing only within themselves. Perhaps this is due to the original creation of humanity, on whom God bestowed the image of God. God has given us unilaterally and unconditionally part of Himself. It is this image of God that allows us to see and seek God; it is the image of God that allows us to continue existing even outside of His Presence; it is the image of God that causes the torment and anguish of Hell, of an eternity separated from Him yet desiring the completion of His Presence and aware of the distance and pain between oneself and that completion. The fires of Hell are the fires of evil, of self-will, and of being divorced from oneself and from God finally and completely and without respite.
And this is mercy.
Only damnation honors us with the dignity of moral will and causation. Universalism conflicts with God's justice and annihilation would defy God's original love in bestowing His image on humble humanity. Hell is the most challenging of all expressions of love and mercy, but it is perhaps the fullest expression of that mercy as well.