It's a fuzzy point for me. While there are no rules for it anywhere the background suggests that it is possible for a creature with Might aligned with one realm to switch to another realm. In the proper set of circumstances fairies will truelly become Infernal or Divine. Apparently Magical Spirits become fae if they get worshiped enough.(Or they can turn into humans find god and get true faith) Of course Angels can certainly fall and switch teams.
It just makes sense to me that Demons might have the same potential.
IMO:
The divine tells us that angels only have the soul for moral behaviour, and the demon the amoral soul, that humans have both (as do Djinns, by the way).
Faeries and magical beings have no souls (except djinns) so lack the moral choice.
Magical beings being worshipped? they will remain magical: they just don't care. A faerie will take their role, mimicking them, and that faerie will become "the magical being worshipped". She remains a faerie.
A faerie might take the role of a demon (the faerie dragon who fight against the faerie archangel MIchel, repeating the famous battle), only because by the story, she can get vitality (here, from the fear of humans reviewing the feared battle of angels vs demons).
If it worked that way, demons would flock to pagan lands and harvest souls without effort. Since they don't, it may be that you have to know that you are sinning for an Infernal aura to appear over time.
Mine where all examples I remembered from various source books. After a little checking I couldn't find the bit I remembered about faeries becoming truly Infernal or Divine so I might be mistaken. But RoP magic does mention at least one situation (story seed on the botttom of pg 108) where a magic being could become fae. Also even if you don't count the devil and most of the other demons as fallen angels there is at least one fallen angel with Infernal might in Ancient Magic(pg 73).
IIRC the correct theory towards pagans was that they were damned. Not because they sinned regularly (that they did) but because they had not been bapthised. As such, they were dirty during all their lives and could not access Heaven no matter what. So, demons do not go there because the souls will go to hell (or purgatory if youy are a good person) without them having to go there to collect them. Better centre your efforts in the competitive market.
You bring up an excellent point about the baptism and purgatory. I can see where there might not be many or any active infernals in Pagan lands, as there would be little (other then to inspire them as tools against the faithful) to be gained from them, but... That still begs the question of infernal auras, which near as I can tell do not require any actual infernal entities' input. From a stylistic perspective, perhaps I should be substituting low level infernal auras where one would normally have low level dominion auras? Then fae or magical auras in actual pagan temples. Since the two can't coexist, I assume even if one interprets the practices of a pagan religion to be sinful, the fae aspect of the religion would win out? Or would that create a regio with a fae side, perhaps in the real world and an infernal regio on the same spot that could be accessed by those of infernal or divine inclination/awareness?
Actually, RoP: Infernal addresses this somewhat. Where you have an Infernal aura and another aura in the same location, the strongest aura is dominant. If the Infernal aura remains weaker then the dominant aura (Magic or Faerie in the case of a pagan site) then the Infernal aura slowly errodes and eventually vanishes... unless it is reinforced by further sinful acts.
Under that logic, a site of pagan sacrifices might have a high Faerie aura with a lower Infernal aura "hiding" within it. The act of pagan worship sustains both auras... which might lead a local demon to try to encourage the sinful aspects of the worship to strengthen the supressed Inferal aura. Further, converting the pagans to Christianity might cause the Faerie aura to fade and vanish, leaving the underlying Infernal aura in it's place.
IIRC, by canon and theology, Angels made the choice to fall and sin when they rebelled and now, this is done and won't happen again: Just as demons can't just quit being demons, angels will no longer rebel versus god.
Still IIRC, this is not as clear cut, in that it presents itself as an angel serving god, while it could be lying, or not, or even deceive itself, but the fact that it has infernal might makes it a demon.
And it changes nothing to the ireedemable/incorruptible aspect of current day demons, angels and metahumans.