theory vs belief

theory and belief both are nouns.

Word	Noun	Adjective	Verb	Adverb
theory	Yes	No	No	No
belief	Yes	No	No	No

As nouns, belief is a hypernym of theory; that is, belief is a word with a broader meaning than theory:

theory (noun)	belief (noun)
a well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena	any cognitive content held as true
a tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena	a vague idea in which some confidence is placed
a belief that can guide behavior