The set up is great I think. The lead character - Cotton Marcus - is quite complex and likeable. He's an evangelical preacher, who has lost his faith. He's basically a decent man, but he's also extremely vain. He's charismatic and he's got used to exploiting less intelligent punters, but he is aware that the phoney 'exorcism' business has led to deaths of so-called possessed children, and doesn't want to be involved any more. Now he's struggling to make a new life, and a living for his family, by exposing religious extremism. The film is the rough footage of a documentary he is shooting, in Louisiana, with a small TV company. I think it's a strong premise.
A widower has written to Cotton asking him to exorcise his 16 year old daughter, who is possessed by a demon. Cotton imposes on this naive man, running a phoney exorcism to demonstrate how easy it is to exploit the gullible. You will not be surprised to learn that things then get a lot more heavy-duty, and the hubris and compassion of the TV crew and the phoney preacher won't let them run away. They are pretty sure the problem is incest, superstition and ignorance. I think it's impossible to say what is 'really' happening in this family because the last few minutes, which supposedly answer your questions, seem to have been grafted on from a completely unrelated film, and should be ignored. I would prefer a film which didn't give an answer.