Hellfire
The Bible teaches of a lake of fire (Revelation 20:15), which is not "sheol" or "hades", but "gehenna" (Luke 12:5, Mark 9:43-48), where the lost will be eternally tormented (Revelation 14:10-11).
Some ask how a loving God could allow this. But we must remember that even though God's love is unconditional (Romans 5:8), and he takes no pleasure in the death of the wicked (Ezekiel 33:11), he also is a consuming fire (Hebrews 12:29), willing to make his wrath known eternally (Romans 9:22, Revelation 14:10-11). If we cut ourselves off from the very source of our existence by our own hatred for him then we must bear the consequences: eternal entropy, eternal decay, where the worm never dies and the fire is never quenched (Mark 9:43-48). Why do men have hatred for their God?
One reason hell lasts forever (Revelation 14:10-11) may be because our spirits can't die or be annihilated; note that they are different than our souls and bodies (Hebrews 4:12, 1 Thessalonians 5:23), and come from the indestructible breath of God himself (Genesis 2:7). Also, hell may last forever for the same reason heaven lasts forever: God wants to make both his mercy and his wrath known eternally (Romans 9:22-23, Revelation 14:10-11), and how could we know eternal mercy if there was no eternal wrath (Isaiah 66:22-24), just as how could we know "up" if there was no more "down"?
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Should We Fear God?
We're to fear God's punishment if we do wrong (Luke 12:5), but if we're made perfect in love, it means we're made perfect in obedience (1 John 5:3), and so don't need to fear punishment.
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Death Cast Into The Lake Of Fire?
Some say the lake of fire can't be literal because how could "death" be cast into it? But in Revelation 20:14, I don't believe "death" is an abstract quality, but the agent which causes death. When the disciples told Elisha "there is death in the pot" (2 Kings 4:40), they weren�t saying "there is an abstract quality in the pot," but "there is something in the pot that will be the agent of our deaths should we eat it."
An angel of death was sent from house to house in Egypt in order to slay the firstborn (Exodus 12:23), and there is an angel of destruction named "Destruction" (Revelation 9:11). So when it says Death and Hell were cast into the lake of fire, I believe it means the Satanic angels of Death and Hell were cast into the lake, where Satan himself had already been thrown (Revelation 20:10), for indeed the lake of fire was originally prepared for Satan and his angels (Matthew 25:41).
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Do Fathers Burn Their Children?
Some point out where it says that burning people never entered God's mind (Jeremiah 7:31). But I believe this means it never crossed God's mind to command Israel to offer their children as burnt offerings to him like they were doing to the false gods Baal and Molech (Jeremiah 19:5, 32:35), but once they started this, he turned it into a judgment directly from him (Ezekiel 20:26).
Some point out that human fathers can't burn their children. But there is no human analogy for the relationship between God and man. Yes, we are his children in one sense, but at the same time we are his creatures: he made us from nothing and maintains us in very existence by his Spirit -- an existential dependency which has no analogy in the parent-child relationship, for a child can reject its parents and even kill them and go on living just as before. If there would be any human analogy it would be that between a mother and a tiny fetus within her. If it were possible for this fetus to rebel against her and cut itself off from her, what would its fate be but death?
There is no real human analogy to our relationship with God because he is so far above any parent: he is the infinite God, a consuming fire (Hebrews 12:29), so holy and powerful that he is like the sun: you simply cannot fight against him without burning yourself up.
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The Very Source Of Our Existence
God is is.
"I AM THAT I AM" (Exodus 3:14).
God is being, existence, the source of all existence. "By him all things consist" (Colossians 1:17). "If he gather unto himself his spirit and his breath; All flesh shall perish together, and man shall turn again unto dust" (Job 34:14-15).
If we cut ourselves off from the very source of our existence by our own hatred for him then we must bear the consequences.
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Universal Salvation?
God will have all the men that are to be saved, not one of them will be lost (John 6:39). But many will be lost (Matthew 7:14, Luke 13:23-28). They will suffer eternal torment in the lake of fire (Revelation 14:11, Mark 9:43-48).