At any rate, the question, as I see it, is this: what is it about a given set of beliefs that renders it religious? I suspect it has something to do with the proximity of the adopted belief system to the core values that inform and shape the believer's sense of identity.
That's a trait, but not unique to religion, being swept up in a political or social movement, or in science, might do much the same.
Religion does have some distinctive traits, though. The most obvious being, empirically unverifiable beliefs about one or more deities, spirits, or other immortal forms of consciousness. Causation will usually be attributed to the immortal consciousness(es). There will be teachings about what happens when humans die. Acts will be defined as right or wrong either because the spirits say so, or based on conformity with (alleged) universal laws; Ṛta and karma bind even the gods. There will be rituals, practices, and holidays.
This is a beginning, but to take it further, there needs to be a context. Like, if we're talking philosophically, historically, or as social scientists, we might accept a group as religious, even if the IRS wouldn't dream of giving them tax exempt status. The IRS says they can't be heavily political, or funnel piles of cash/benefits to their leadership, and "the organization’s purposes and activities may not be illegal or violate fundamental public policy." I can't find any sign of FLDS organizations with 501(c)(3) status. No Thuggee Church, or tax-exempt temple prostitution.
Since I live in the US, where the IRS is nearly the sole arbiter of what constitutes religion, and where there is really
no effing chance that churches will be taxed in my lifetime, I have to conclude that scientology is not a religion, and that it was granted 501(c)(3) status due to extortion induced error. The CoS went into court, not once but twice, claiming that fair gaming people was legally untouchable, since it was a core religious belief of scientologists. This is essentially proof, in their own words, that they can never honestly meet the legal criteria for religion in the US. Core beliefs violate fundamental public policy.
So if you're over at your best friend's house, and you're smoking a joint and waxing philosophical, sure, scientology's a religion, just like being a Trotskyite, or attending Landmark Education, or swooning over Ayn Rand. Maybe nationalism's a religion. Maybe my marriage can be a religion. (No thanks, that last hit was plenty.) If it can be, does that mean that all of my descendants can live tax free forever?
And at that point, you've come full circle. In theory, almost anything can be a religion. From a pragmatic, political perspective, very few beliefs (if any) should be singled out for the special privileges that come with that label.