The most terrifying doctrine in the Bible
There is surely no more terrifying, no more serious, no more difficult doctrine in the Bible than its teaching on hell - a place of eternal, conscious suffering, for those who reject the God they know exists from creation, or worse - reject the Jesus who they have heard about. No Christian should speak about hell without sadness and even tears.
In every age, this difficult truth will be undermined, spoken against, revised and denied - but there are particular reasons this truth will be denied today. First, we live in an easy world. By that I mean, we, speaking of the West, find ourselves in comparative comfort and ease. Any doctrine which implies discomfort will naturally be alien to us. Second, we are a minority-rights culture, where the thought of any group being treated poorly sounds just wrong to a modern ear. Third, we are a rationalistic culture, in which the mind is king - this doctrine does not seem reasonable to the human mind (which is why liberals have denied it for yonks).
Reasons to believe in hell
I believe in hell, because (1) it runs against the natural mind of fallen human beings. This may seem to be a strange reason, and it's not a main one, but so much of God's truth is counter-intuitive, supra-intellect, beyond our ability to understand, that if a doctrine is automatically denied because it fights against reason, that is a reason to suspect that it may be true; for who can know the mind of God? If all a man believes is understandable to his intellect, his doctrine is very unlikely to have a divine origin. This is not to say that we should believe any doctrine that is irrational, but one good reason to consider as true a well-established doctrine, is that the small feeble and fallen human mind fights it.
I believe in hell because (2) it supports the justice of God. A God who allows Hitlers and Stalins to get away with monstrous sins, indeed allows any sin to be overlooked is not a just God: such a "god" is a monster - overlooking evil and wrongdoing.
I believe in hell because (3) I believe in the cross of Jesus Christ. The suffering of the Son of God who became sin for us and was therefore punished on the cross only makes sense if you believe that God is a just God who must punish sin. If God can overlook sin, if he can turn a blind eye to sin, if there is no hell, why didn't he overlook all sin and spare his Son from Calvary? Why did he send his Son to redeem us at such great cost if he could have simply overlooked sin?
I believe in hell because (4) I believe in evangelism and mission. There is, frankly, no point in sharing the Gospel around the world, if everyone is automatically going to heaven. Why waste our time - indeed this is why liberals don't bother with the Gospel, why upset people if there is no need to? Just feed their bodies, don't bother with their souls.
I believe in hell because (5) the Bible teaches it. Now of course we are getting closer to the core reasons. The only way to deny the Scriptures' plain teaching about hell is rip out considerable chunks of the Bible or do one of those pathetic revisionist interpretation. Take one example alone. Paul says to Christians who are being persecuted that one day God, "will punish those who do not know God and do not obey the Gospel of our Lord Jesus. They will be punished with everlasting destruction and shut out from the presence of the Lord..." (2 Thess 1:8-9). No amount of liberal hickledy pickeldy pockery "this doesn't mean that, that doesn't mean this" magic can alter the plain meaning of the words "punished", "everlasting", and "destruction".
I believe in hell, most of all, because (6) Jesus, the Son of God, the lover of souls, taught it. In fact, it is a well-known and well-quoted statistic, that Jesus taught more about hell than anyone else. I believe Jesus when he says: "But the subjects of the kingdom will be thrown outside, into the darkness, where there will be weeping and gnashing of teeth." (Matt 8:12), and "It is better for you to enter life maimed than to have two feet and to be thrown into hell" (Mark 9:46), and "In hell where he was in torment, he looked up and saw Abraham.." (Luke 16:23) and, "Then they will go away to eternal punishment, but the righteous to eternal life" (Mat 24:46).
The essential message of the Gospel is "flee from the wrath to come"; come to Christ who has paid hell for us, so that we don't have to bear it ourselves and instead become the recipients of the amazing grace of God.