Can someone tell me about the fire sermon?

That everything, concepts/perceptions/passions, will become exhausted. (That was a short and bad answer, sorry) Better just to experience reading it:

http://www.budsas.org/ebud/ebsut026.htm

Of course, someone will try to compare the title to hell :(

It helps to understand that the Buddha was a very effective speaker who's sermons often targeted specific audiences. In this case he was addressing a group of ascetics who were practitioners of a Vedic fire ritual.

Since the Buddha wanted to reach them, he thought that fire would be a suitable analogy to capture their attention. So he applies the idea to the "six sense bases." That is, the five senses as we know them plus the mind.

By aflame, he is speaking of greed, anger, and delusion. Greed anger and delusion arise based on the six sense bases.

When the Buddha spoke to ascetics, he assumed that they were aiming for liberation in this lifetime - so he went straight for the gut when speaking of what he termed the "dangers of sensual pleasures."

This language seems a bit extreme to us, but realize that in this case he was speaking to ascetics. He did not typically speak in this way to householders, to whom he generally taught more easy-to-swallow sermons concerning general morality and behavior that would lead them towards a better future or better rebirth.

I don't believe God is so heartless that he will let people burn in Hell forever. I believe there might be a Hell, but I doubt any humans have ever gone there, or ever will.

No human being deserves that kind of torment.

God would have to be heartless to just leave them there...