From this and other similar works such as Terry Pratchett’s Small Gods or Neil Gaiman’s Ameeican Gods, deities pop into existence from the pure belief of people. It’s a great literary conceit.
Man is certainly stark mad: he cannot make a worm, yet he will make gods by the dozen.
Michel de Montaigne, 1533-1592.
(Still, Small Gods is one of the best revisions of the topic!)
I can't think of any religion where pure belief of people cause deities pop into existence. Unless of course you are calling the fact that people talk about the deity as popping them into existence. In that case, you can say that JK Rowling caused Voldemort to pop into existence (how evil of her).
If on the other hand you start with the premise that there is a magical world created somehow then lesser gods can exist and cease to exist for all manner of reasons. We can apply it to the real world in the context of cultural memes. J.K. Rowling didn’t cause Voldemort to pop into existence, the fans of the books and those who got hysterical over depictions of witchcraft did.