It seems that the biggest hang-up people have with the concept of hell is that it is eternal. The argument goes like this, "It doesn't seem fair that someone who sins for a relatively short time on earth should have to pay for their sins by being eternally punished in hell." This is an important concern for us to consider. Why is hell eternal?
I want to offer three lines of thinking in answer to this question. The first comes from C.S. Lewis via Tim Keller. The other two are more original (though I believe they have the support of Scripture, and others have probably formed similar thoughts). At the same time, I'm still processing them, so any thoughts you have would be helpful. You can comment by clicking "Discuss this Post" below.
1. A relational explanation (Lewis/Keller).
This explanation basically goes like this: At some point our body will die on this earth, but our soul lives on into eternity to experience what we have desired and lived for in this life. So if you desire Christ and trust him as your Savior, that is your eternity. If you rejected your Creator and want to live without him here, you will do so for all eternity. There's a lot more to this, but you'll need to read chapter 5 of The Reason for God for more.
2. A better understanding of justice.
We understand the concept of justice and punishment for people who are unjust, for people who do "bad" things. But if we are able to judge them for that - if we find it acceptable to pronounce judgment on someone else for being bad, that means we feel we are morally superior to them. That is, our morality condemns them for being less moral. At the same time, we realize that we are not perfectly moral. We know that we probably deserve some punishment for bad things we have done. But still, we feel like we can say that Hitler or a rapist or someone who hurts us should receive punishment because we are at least a little bit more moral than them.
Now, take that same concept and apply it to God. While we are somewhat moral (and therefore feel just in pronouncing judgment on others that we deem less just), God is infinitely just. He is righteous and just to a degree that we cannot fathom. And if we cannot fully fathom his righteousness and justness, then we also cannot fathom our injustice, badness, and violation of his rightness.
This is another reason that hell is eternal. God is infinitely just (beyond our understanding), and we are infinitely wicked. And this is exactly why the cross is so awesome. Jesus paid the price we couldn't pay. He paid the price we cannot even fully understand. But we can understand it a little bit because we operate the same way with people we deem to be unjust. We just need to take that concept and magnify it a million times to realize what is going on between a holy God and sinful people.
3. A proper understanding of "paying for sins."
If hell was temporary wouldn't that mean that people are basically "paying off" their sins? But the Scriptures are clear that we can't pay for our sins - only Jesus can. Further, it would seem that if it were possible to pay for sins, then the sinner should be remorseful. Like a serial killer would need to repent for his murdering in addition to serving a ton of time in prison. But even then, could that really pay for his crimes? Normally our society doesn't think so because we put serial killers in prison for life. What we are saying is that there is no way he can ever pay for his crimes - therefore the best we can do is have him spend his whole life in prison or be executed. No one believes that his time in prison actually pays for his crime, but it is the worst sentence we can come up with. Likewise, hell does not pay for sin. Even though hell is eternal, not one sin has been paid for because apart from Jesus there is no repentance and no possible payment.