I'll briefly examine here what I believe the Bible has to say about hell. Honestly, I think it's really clear; I think that people don't like the idea of hell, so they try to remake it into annihilation or into a fantasy where everyone goes to heaven. These fly in the face of God's justice, holiness and perfection.
Hell was not made for people, but for the rebellious angels who rejected the Lord and were cast out of heaven (they know they are going there at the appointed time):
Matt 25:41 (Jesus talking)
"Then he will say to those on his left, 'Depart from me, you cursed, into the eternal fire prepared for the devil and his angels."
2 Peter 2:4-10
For if God did not spare angels when they sinned, but cast them into hell and committed them to chains of gloomy darkness to be kept until the judgment; if he did not spare the ancient world, but preserved Noah, a herald of righteousness, with seven others, when he brought a flood upon the world of the ungodly; if by turning the cities of Sodom and Gomorrah to ashes he condemned them to extinction, making them an example of what is going to happen to the ungodly; and if he rescued righteous Lot, greatly distressed by the sensual conduct of the wicked (for as that righteous man lived among them day after day, he was tormenting his righteous soul over their lawless deeds that he saw and heard); then the Lord knows how to rescue the godly from trials, and to keep the unrighteous under punishment until the day of judgment, and especially those who indulge in the lust of defiling passion and despise authority.
When he arrived at the other side in the region of the Gadarenes, two demon-possessed men coming from the tombs met him. They were so violent that no one could pass that way. 'What do you want with us, Son of God?' they shouted. 'Have you come here to torture us before the appointed time?'
Hell is eternal:
Mark 9:42-48 (Jesus speaking)
"And if anyone causes one of these little ones who believe in me to sin, it would be better for him to be thrown into the sea with a large millstone tied around his neck. If your hand causes you to sin, cut it off. It is better for you to enter life maimed than with two hands to go into hell, where the fire never goes out. And if your foot causes you to sin, cut it off. It is better for you to enter life crippled than to have two feet and be thrown into hell. And if your eye causes you to sin, pluck it out. It is better for you to enter the kingdom of God with one eye than to have two eyes and be thrown into hell, where
" 'their worm does not die,
and the fire is not quenched.'"
And another angel, a third, followed them, saying with a loud voice, "If anyone worships the beast and its image and receives a mark on his forehead or on his hand, he also will drink the wine of God's wrath, poured full strength into the cup of his anger, and he will be tormented with fire and sulfur in the presence of the holy angels and in the presence of the Lamb. And the smoke of their torment goes up forever and ever, and they have no rest, day or night, these worshipers of the beast and its image, and whoever receives the mark of its name."
It probably needs to be reiterated about the former passage that Jesus is NOT talking about literally cutting off your hand, foot, or gouging out your eye. While he is making an analogy here, I don't believe the analogy extends to hell, a place Jesus referred to often in his teachings.
I think of this one often:
And do not fear those who kill the body but cannot kill the soul. Rather fear him who can destroy both soul and body in hell.
Jesus spoke to the Pharisees about where their practices and hearts were leading them. This passage flouts the teachings of those who want to present Jesus as a frail teacher who would never speak a harsh word to anyone. With many sinners, He was gentle, but He was angered at the hypocrisy of the teachers of the Law who were leading the people astray. The passage, and the one below it, also demonstrate that hell is the place the rebellious are sentenced to at the Judgment:
"Woe to you, scribes and Pharisees, hypocrites! For you build the tombs of the prophets and decorate the monuments of the righteous, saying, 'If we had lived in the days of our fathers, we would not have taken part with them in shedding the blood of the prophets.' Thus you witness against yourselves that you are sons of those who murdered the prophets. Fill up, then, the measure of your fathers. You serpents, you brood of vipers, how are you to escape being sentenced to hell?"
"When the Son of Man comes in his glory, and all the angels with him, then he will sit on his glorious throne. Before him will be gathered all the nations, and he will separate people one from another as a shepherd separates the sheep from the goats. And he will place the sheep on his right, but the goats on the left. Then the King will say to those on his right, 'Come, you who are blessed by my Father, inherit the kingdom prepared for you from the foundation of the world. For I was hungry and you gave me food, I was thirsty and you gave me drink, I was a stranger and you welcomed me, I was naked and you clothed me, I was sick and you visited me, I was in prison and you came to me.' Then the righteous will answer him, saying, 'Lord, when did we see you hungry and feed you, or thirsty and give you drink? And when did we see you a stranger and welcome you, or naked and clothe you? And when did we see you sick or in prison and visit you?' And the King will answer them, 'Truly, I say to you, as you did it to one of the least of these my brothers, you did it to me.'
Then he will say to those on his left, 'Depart from me, you cursed, into the eternal fire prepared for the devil and his angels. For I was hungry and you gave me no food, I was thirsty and you gave me no drink, I was a stranger and you did not welcome me, naked and you did not clothe me, sick and in prison and you did not visit me.' Then they also will answer, saying, 'Lord, when did we see you hungry or thirsty or a stranger or naked or sick or in prison, and did not minister to you?' Then he will answer them, saying, 'Truly, I say to you, as you did not do it to one of the least of these, you did not do it to me.' And these will go away into eternal punishment, but the righteous into eternal life."
More passages that demonstrate the reality and nature of hell are below. In Luke 16 Jesus talks about Lazarus and the rich man:
"There was a rich man who was dressed in purple and fine linen and lived in luxury every day. At his gate was laid a beggar named Lazarus, covered with sores and longing to eat what fell from the rich man's table. Even the dogs came and licked his sores.
The time came when the beggar died and the angels carried him to Abraham's side. The rich man also died and was buried. In hell, where he was in torment, he looked up and saw Abraham far away, with Lazarus by his side. So he called to him, 'Father Abraham, have pity on me and send Lazarus to dip the tip of his finger in water and cool my tongue, because I am in agony in this fire.'
But Abraham replied, 'Son, remember that in your lifetime you received your good things, while Lazarus received bad things, but now he is comforted here and you are in agony. And besides all this, between us and you a great chasm has been fixed, so that those who want to go from here to you cannot, nor can anyone cross over from there to us.'
He answered, 'Then I beg you, father, send Lazarus to my father's house, for I have five brothers. Let him warn them, so that they will not also come to this place of torment.' Abraham replied, 'They have Moses and the Prophets; let them listen to them.'
'No, father Abraham,' he said, 'but if someone from the dead goes to them, they will repent.' He said to him, 'If they do not listen to Moses and the Prophets, they will not be convinced even if someone rises from the dead.'
Again, hell has much to do with God's judgment--His place as the just Judge. His heart is with the oppressed and poor, and those who have been taken advantage of will be avenged by Him:
Your wealth is rotting away, and your fine clothes are moth-eaten rags. Your gold and silver have become worthless. The very wealth you were counting on will eat away your flesh in hell. This treasure you have accumulated will stand as evidence against you on the day of judgment. For listen! Hear the cries of the field workers whom you have cheated of their pay. The wages you held back cry out against you. The cries of the reapers have reached the ears of the Lord Almighty.
Greg wrote something I agree with in the comments at EU (below). I don't go around trying to evangelize by telling people they're heading for hell if they do not repent, unless the Spirit prompts me to do so, which is rare for me personally. I support preachers who teach about hell, though--they should. Jesus mentioned it quite often. I think that we should be honest about what hell is and why people go there. Jesus said it in a passage above: those who are consigned there are workers of lawlessness, who refused to repent and obey God.
Romans 1 lets us know that everyone is without excuse. I've realized after lots of conversation with many atheists that they are angry with God in some way--it's not so much that they totally can't conceive of the notion, though that's what they say at first. The idea of hell, or something hurtful that has happened to them or to a loved one has made them angry with God...and that's a far cry from really not believing He exists.
"I don't think anyone is going to "come to faith" by having their understanding of Hell clarified. It is not the removal of mental obstacles that brings us to faith and repentance. It is a terrifying encounter with God’s holiness, the devastating realization of my own sinfulness, and the awesome reality of God’s free gift in Jesus Christ.
Hell makes perfect sense in light of God's holiness. His holiness is not His majesty; it is that which completely separates us from Him in our (un)natural condition. Chris, you said, 'We're trying to understand how/why someone would be condemned there,' and that is precisely what I am getting at by saying we must understand His holiness before we will understand Hell."