>>96364684 (OP)
Since the obvious inspiration for this question is Christianity vs Paganism, you should know a few things:
- There's no reason from a Christian perspective to deny that the followers of the pagan religions have the ability to magically do things
- This is because they are empowered by demons pretending to be their false gods. Demons are happy to pretend to be whatever they want to your face, so long as they think you'll believe them, which is why they pretended to be pagan gods in ages past, and MLP tulpas in the modern day.
- However, if you read the hagiographies of Saints coming up against pagan magicians and such, or of Moses going up against the Pharaoh's court priests, the magicians and demons empowering them are simply outclassed by the power of the one true God and it's no contest.
- However, whether someone that isn't a Saint can be a vessel for God to completely BTFO demonic magic is a crapshoot, since sin opens you up to being under the metaphysical jurisdiction of demons, which enables their magic to work effectively on you in the first place
This is likely to not be what you're looking for for a custom tabletop RPG setting, But it could give you some further ideas. Like instead of the other false gods being just demons, they could be like the relation of the Ainur to Eru Illuvatar in Lord of the Rings, with Melkor being a fallen Ainur. Then you could possibly have various Your!Ainur either rebelling against or being in favour of Your!EruIlluvatar.
>>96364733
>>96364710
You're using, at best, an extremely idiosyncratic and strained definition of "monotheistic."