I can't recall what JB's denominational exposure is/was, but it may have been Catholic.
The only thing I recall Jim saying is he was from the "fun" end of fundamentalism spectrum. Catholics are not typically considered fundamentalists, so he probably wasn't Catholic.
We don't know if Mac's significance changed as he wrote, like Butters' did.
I think Mac was important pretty early on. He's constantly described as fitting in the same age range. That was the first thing I noticed that made me think something was up with him.
But I also swear that I saw or heard a WoJ (that I have been unable to find, so it's not supported) that Uriel *believes* he was present at the creation of everything, but that doesn't make it *true*. The implication seemed to be that Uriel is shaped, even beyond his own awareness, by what human religion established for him and the other angels.
I don't know if that was JB hedging to avoid offending those with different beliefs, or if it was an early mental draft to what he was developing for the story, or if that's an accurate description of the Dresdenverse.
I recall Jim saying something like the Creator could have just retconned reality to make Himself the Creator and how would anyone know the difference. For the italicized portion, I feel like that was what Jim was doing, but that wouldn't make the bolded portion false either.
And personally while I support totally supes getting high and mighty on spiritual or psychic power of faith of believers, I'm really tired of trope that gods/titans/faeries are just some manifestations of human belief and do not exist really beyond that.
I think it's played a little different in the DF.
It could be that it's all about will. We've seen older beings with will so powerful that it flattens mortals because the will of a few (or maybe hundreds) of mortals can't compare, but the problem for these older beings is that there are billions of mortals. When the will of a billion mortals is that you're just a story, it probably has an affect on you. When you have a bunch of mortals devoted to you being all powerful or just really powerful, that also probably has an affect on you.
Or it could just be that mortal knowledge anchors these beings to reality. (I think your "supernatural beings do not change their natures" WoJ supports this one).