(Everything I’m saying here is based on experiences in a few religious studies classes I’ve taken, some papers that I’ve read, and some conversations with religious studies people. The things I say might not be actually be representative of the aggregate of religious studies scholars, though Google Scholar would seem to provide some evidence for it.)
Religious studies people tend to put a lot of emphasis on the fact that ‘religion’ is a fuzzy word. That is, while there are some organizations that everybody will agree are religions (Judaism, Christianity, Islam), there are edge cases that are less clear (Unitarian Universalism, Hare Krishnas, Christian Science). In addition, attempts to lay out a set of necessary and sufficient conditions for membership in the category “religion” tend to either let in too many things or not enough things.
For some reason this is taken to be a very significant fact, and people solemnly intone things like “Is nationalism a type of religion?” and “Isn’t atheism really just the new popular religion for the young?”. Sociologists spend hours arguing with each other about different definitions of religion, and invoking new typologies to distinguish between religions and non-religions.
The strange thing about this is that religion is not at all unique in this regard. Virtually every word that we use is similarly vague, with fuzzy edges and ambiguities. That’s just how language works. Words don’t attain meanings through careful systematic processes of defining necessary and sufficient conditions. Words attain meanings by being attached to clusters of concepts that intuitively feel connected, and evolve over time as these clusters shift and reshape themselves.
There is a cluster of important ideas about language, realization of which can keep you from getting stuck in philosophical dead ends. The vagueness inherent to much of natural language is one of these ideas. Another is that semantic prescriptivism is wrong. Humans invent the mapping of meanings to words, we don’t pluck it out of an objective book of the Universe’s Preferred Definitions of Terms. When two people are arguing about what the word religion means, they aren’t arguing about a matter of fact. There are some reasons why such an argument might be productive – for instance, there might be pragmatic reasons for redefining words. But there is no sense in which the argument is getting closer to the truth about what the actual meaning of ‘religion’ is.
Similarly, every time somebody says that football fans are really engaging in a type of religious ritual, because look, football matches their personal favorite list of sufficient conditions for being a religion, they are confused about semantic prescriptivism. At best, such comparisons might reveal previously unrecognized features of football fanaticism. But these comparisons can also end up serving to cause mistaken associations to carry over to the new term from the old. (Hm, so football is a religion? Well, religions are about supernatural deities, so Tom Brady must be a supernatural deity of the football religion. And religious belief tends to be based on faith, so football fans must be irrationally hanging on to their football-shaped worldview.)
It seems to me that scholars of religious studies have accepted the first of these ideas, but are still in need of recognizing the second. It also seems like there is a similar phenomenon going on in sociological discussions of racial terms and gender terms, where the ordinary fuzziness of language is treated as uniquely applying to these terms, taken as exceptionally important, and analyzed to death. I would be interested to hear hypotheses for why this type of thing happens where it does.