You guys should watch "The Invention of Lying." He tells his mom who is afraid she'll go to nothingness, "you're gonna go to a better place, which has mansions." Though technically, her guess that there'd be nothing there is no more the truth than anything else, just that a culture that has no deceits, has no sense of the uncertain, so it was not so much a lie as a "your guess is as good as mine."
Stripped of all assumptions, I'd guess religion, in terms of afterlife goes something like this:
- You body decays much faster when dead than alive. (Read up on it, without emblaming chemicals half of it degrades within a week or so)
- Therefore, something inside either leaves (we can call it a "soul" or anima), or converts from matter into energy (causing radioactive decay or something)
- If it leaves, it must go somewhere, even if it's just back to the soil (spiritual recycling) or perhaps there is some place for it to go for awhile.
- If it's transformed, it either gets spent to power new lifeforms ("matter can neither be created nor destroyed" so the soil as the body decays grows plants, which are then eaten by critters, which use the energy to birth calves or children)
- Assuming that a soul is different from conventional matter, possibly there's a not-strictly-physical place for this to go (if for instance, a soul is more memories and such than potential/kinetic energy, it would go into a sort of pool that could roughly be defined as an afterlife)
- Such a place, assuming it existed, would be pretty remote from conventional Heaven or Hell, but rather a sort of Grey Waste where people retain whatever experiences they went in with (so if you weren't popular on Earth, that might be unlikely to change, as is a alcohol problem likely to give you the spiritual echo of perpetual stomach ailments) In this case, it's neither good or evil, but bad memories can make it a "Hell", as good ones might make it like "Heaven."