I liked Harari’s definition of a religion in Sapiens: a belief in a superhuman order that prescribes norms for human behaviour.
So the general theory of relativity is not a religion, because while its laws are not believed to be of human origin (i.e., they would still hold whether or not humans ever existed), they do not prescribe human behavioural norms. Association Football, conversely, is not a religion because its laws, although governing human conduct (e.g. do not become involved in active play when in an offside position), are not held to be of non-human provenance (usually, anyway).
Roman Catholicism is a religion, because its laws are held to be of superhuman origin (specifically, divine revelation) and do set rules for human behaviour (e.g. no wanking).
Note that this definition covers institutions and ideologies not typically considered religions, such as capitalism. For Harari, this is a feature, not a bug.
