A generic definition. Looked this up from an erroneous reference to Émile Durkheim via a video from Henrik Jönsson dealing with why the Environmental Movement has become a religion:
A religion is a unified system of beliefs and practices relative to sacred things, i.e., things set apart and forbidden – beliefs and practices which unite in one single moral community called a Church, all those who adhere to them.
This quote from Infogalactic continues:
In this definition, Durkheim avoids references to supernatural or God. Durkheim argued that the concept of supernatural is relatively new, tied to the development of science and separation of supernatural—that which cannot be rationally explained—from natural, that which can. Thus, according to Durkheim, for early humans, everything was supernatural.
Thus, it seems a good definition to use when trying to pigeonhole a political/social religion like environmentalism or leftism.