Pretty much this along with the facts that they don't worship an unexploded bomb, follow the master, believe we are plagued with neurodynes, etc.
You're saying the beliefs/practices are weird, or abnormal, in certain (or sometimes, I'm guessing, all) ways. And this was one point where I
think I'd maintain my own current beliefs and absence of organized religion-because I'd agree, some of the stuff is far out there(and, in at least some of the cases I know of, a tad too destructive/hopeful for destruction for my tastes). But don't forget that in society all religions/ideologies/beliefs, organized or not, tend to have "quirks" or "abnormalities" of some sort-we just accept them as the norm. Likely people born into a situation would act similarly-sure some would genuinely find it all very stupid and/or pointless, but it would be likely there'd also be some who were completely fine with it/agreed with it. Plus, ignorance would be more predominant in society most of the time, so common sense would be less likely to enter the picture as often as you'd think it ought to. (Not trying to talk about actual religions here, just making a point on beliefs existent in the wasteland and how people might look at it in their current situations and environments, based on human behavior and such. =])
Though I always thought many of the religions sprouted up in the Fallout universe were sort of mock-religions for comedic effect alone, and weren't really meant to be taken seriously. Not enough to even consider having a character of a player's worship those ones. Though I don't really know the ones from Fallout 1 or 2, aside from that their names are vaguely familiar to me. =\
And the Master wasn't entirely insane in his actions, from the little I know about the original Fallouts. His idea had SOME reasoning/logic behind them, he just didn't know mutants are sterile and such. And was, to say the least, a bit extreme.
If I was born into the wasteland, rather than just surviving the Great War(in which I doubt I'd have the logic of "becoming spiritual= guaranteed to save my soul/rump," as I'm a bit of a skeptical person), I'm not really sure what I'd be. Probably whatever I was brought up to believe-and if that was nothing in particular, I'd probably either go with whatever seemed to be the most productive, or be totally atheist. It seems, often, people go to the extremes of spirituality in really tough situations-usually resorting to no or crap loads of religious dedication. I really doubt I'd be so special as to not fall in with the typical crowd, unless my hypothetical parent(s)/guardian(s) encouraged being open-minded about the possibilities or somethin'. xd I guess in the case of religion, then, I'd go with the Followers of the Apocalypse? Again, don't really know much about them, but they seem a hell of a lot more productive/sensible than, say, the Children of Atom. Who most, I think, would agree is a bit too crazy for even the more desperate of people.