a question for world-building religions

and a selection of answers.  Not all of which I agree with, but I think that here is treating one of the right questions to ask.

riveting

If there's a zeppelin, it's alternate history.  If there's a rocketship, it's science fiction.  If there are swords and/or horses, it's fantasy.  A book with swords and horses in it can be turned into science fiction by adding a rocketship to the mix.  If a book has a rocketship in it, the only thing that can turn it back into fantasy is the Holy Grail.

Debra Doyle

(My own rule of thumb used to be what made SF was the rivets -- if it had 'em, it was SF -- but I'm working on a fantasy steampunk Ruritanian romance and consequently had to discard that one.)