Because hellfire means the urge to do the evils prompted by love for oneself and the world, and because this kind of urge is characteristic of everyone in hell (see the preceding chapter), when the hells are opened one sees the kind of smoky fire you get in conflagrations. An intense fire rises from the hells where love for oneself rules, and a flickering one from the hells where love of the world rules.
When the hells are closed, though, this fire is not visible. Instead, there is something dark, as though made from thickened smoke. Still, that fire is raging inside, as you can tell from the radiating heat. It is like the heat of ruins after a conflagration, or in some instances like the heat of an active furnace, or in others like that of a hot bath. When this heat flows into us it rouses our cravings—hatred and vengefulness in evil people, and dementia in the ill.
This kind of fire or heat is characteristic of people who are engrossed in the aforementioned loves because in spirit they are in touch with these hells even while they are still living in their bodies.
It does need to be known, though, that the people in the hells are not actually in fire. The fire is an appearance. They do not actually feel any burning, only the kind of warmth they knew in the world. The reason there seems to be fire is correspondence, since love corresponds to fire, and everything we see in the spiritual world has its visible form from its correspondence.
We should bear in mind that this hellish fire or heat changes into an intense cold when warmth flows in from heaven. Then the people there shiver as though they had feverish chills and are in inner torment. This is because of their absolute opposition to the Divine; and heaven’s warmth, which is divine love, snuffs out the warmth of hell, which is love for oneself, and along with it the very fire of their life. This is what causes the cold, the shivering, and the pain. It also then brings about darkness there, and folly and confusion. This does not happen very often, though; only when there is need to quell rebellions that are getting out of hand.
Since hellfire means all the craving to do evil that flows from love for oneself, that same fire also means the kind of torment that occurs in the hells. This is because the impulses that arise from that love are urges to wound people who do not offer respect and deference and reverence. To the extent that rage takes charge, and the hatred and vengefulness that come from rage, people are driven to attack others viciously. When this impulse is inherent in everyone in a community where there are no external restraints, no fears of the law or of loss of reputation or position or profit or life, everyone attacks everyone else out of sheer malice. The strong conquer and subject the rest to their tyranny, cheerfully torturing any who do not surrender. This latter delight is integral to the delight in tyranny to the point that they are of equal intensity. This is because sadism is inherent in hostility, envy, hatred, and vengefulness, which as already noted are the evils of a love of cruelty.
All the hells are communities like this, so everyone there cherishes hatred toward others in her or his heart and bursts out in savagery whenever there is the strength to do so.
These acts of savagery and torture are what are meant by hellfire, because they are the results of their obsessions.
from Heaven and Hell, Sections 571-573