Neither a prophet nor scripture is a necessary component of religion. The classic religions of the Greeks, Romans, Germanic tribes, Africa, and the New World, had neither of these things, and the list is not exhaustive by any means.
If one were to talk about religion in general, the definition would probably be close to encompassing nation states and corporations too: an intersubjective idea, that exists as long as people believe in it, and that evolves independently of any individual or group of believers.