Not that I think Religion is going to 100% be the focus, but if that was going to be the case, I would suspect that the war would not be over followers in the normal sense. I could see Daimon priest getting special abilities like raising cultist army's etc, and perhaps something along the line of the Light Temples to provide realms a way of countering the religious spread without relying solely on priest. If you think outside of the current priest game, things could be added to either side to provide and interesting religious war.
Indeed, it could, but it would feel truly bizarre for the humans to have special items/powers to counter daimonic religious spread. Despite the daimons and other invasion, BM remains a rather low-magic setting, where most things magic are rather limited. The Light, for example, only had 3 scattered temples. If every realm was to have one, it would have given quite a different tone to the universe. Of course, part of the struggle could be that the items required aren't in large enough numbers for every realm to have one.
My imagination isn't god-like, there can obviously be things I'm not thinking of. Story-wise, having the daimons turn the peasants into fanatical cultist armies would be interesting. Gameplay-wise, mechanics to counter this would either have to rely on too few people (priests) or too much magic (enough items/powers for most realm to somewhat limit daimonic mind control).
