I’m not really a fan of horror movies, and I’m utterly ignorant of religion, so I’m not going to discuss theist/atheist things in details with you. But I think your premiere is a bit not-so-original. I mean, the idea of a movie crew being terrorized by supernatural beings is pretty much used. We have Grave Encounters, Skeleton Crew, Diary of the Dead and even Sweet Home. So it may not attract the audience at first sign.
I know you have it planned in mind, but I don’t really get something in your plot: why do demons only appear when the movie crew is filming? I mean, they are atheists, so they have lost their faith (forgive me for this statement), so the demons must have been terrorizing them for a long time, ever since they became atheists. Why do they just wait until the movie crew start filming then terrorize? Are you suggesting that the movie crew used to be theists, then became atheists shortly before they shoot the movie?
The theist/atheist thing is making the plot complicated, as many have stated above. I think it would be better if the plot goes like this (sorry, suggestion of a detective fan): some people (or all) of the movie crew have done something wrong some time before the movie shooting (maybe together) and the movie accidentally has a lot in common with that incident. The demons are actually the spirits of the dead (if the movie crew murdered someone), and/or their conscience tormenting them.