When is a Religious Belief Delusional?

[Whateverist]

I guess you could compare and contrast different types of beliefs.

The kind of theist who mostly accepts mainstream science, but just shoehorns in "but God did it", isn't operating under a delusion as such. It's just forcing a conclusion which probably won't affect them much.

The kind of theist who prefers their literal stories to mainstream science is pretty delusional, I would say. They're living in a borderline fantasy land version of reality. This still might not impact them too much in everyday life, although I find it hard to believe that this kind of thinking won't have some knock-on effects.

I agree.  Literalists continually reject the real in favor of their cherished fantasy.  That does seem outright delusional to me too.