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Almost all critics are trying to bury this movie because it is right-wing and Christian (all the other left-wing anti-Christian movies were never a problem though).
The movie is a cheap production, all the major plot revolves around two guys chatting in a room, but the dialog is really well written and both actors are doing a great performance.
It is also very different from modern horror movies, it does not contain jumpscares and shaky cameras. It would probably be better classified as a psychological thriller.
Even if you are not religious but have some interest in Christian lore you will probably enjoy it.
My one complaint with the movie (from a film/storytelling standpoint) is that it would have been a better film if it ended a scene earlier, but because of who it is marketed to they have to add a final scene to soften tone of the movie. It also had one or two points that I thought were theologically inaccurate, but it was otherwise a pretty decent movie.
What parts were those?
I don't recall specifically. Something about the nature/person of God and/or the power/role/creation of angels/demons probably.
I think the movie is based on thomistic interpretation of Lucifer's revolt wich is very interesting, but some protestants may not find it completely "biblical." Some calvinists may even disagree entirely.