I had been more or less assuming that all of the religions were true, so to speak. In the past two games we see physical manifestations of many of the Nine Divines; even Talos, and if any of the 9 were false it would presumably be him. Given that kind of powerful evidence being available, I had assumed that the Imperial pantheon would already have superseded the others unless they, too, had been manifested in like fashion, or otherwise been definitely proven.
From what we've seen, the Aedra (and Lorkhan) are retroactive; mortal myths shape them, and they can be many different things at the same time to different groups. Given the Daedra don't seem to conform to myths, I suspect that this is due to the Aedra taking part in the "Divine Pact".
In any case, I'm mostly looking at this from a role-playing perspective. From what you've said, it sounds like a non-Imperial following the Nine Divines would (in the eyes of most) be a conversion of faiths, and presumably only those who have abandoned their own cultures to some degree would do it. The question this raises in my mind is, why is the Imperial Cult the only widespread religion? Well, it's probably because the developers didn't want to have 10 religions in each area. But is there a more lore-friendly response? I.e., is Altmeri (or Khajiiti, or whatever) religion more personal and private than Cyrodiil, and hence there is no need for their own temples? They can't all be agnostics.
A lot of the cultures have their own pantheons, although most of them are primarily Aedric. The Dunmer, however, have no history of Aedra worship, preferring to worship the Tribunal, Daedra, and their ancestors (which they believe the latter to be). From what I've seen so far, the Imperial Cult proselytes in places where worship of the Divines is less widespread. So how much activity they would have in other provinces, we don't know for sure.
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