The fact that he does not trust doctors influences him to search for help with an exorcist and reveals the true magnitude of the problem. As a parable, the Last Exorcism is meant to have people understand the importance of remaining faithful regardless of the problems that they come across. Furthermore, the film is also intended to influence viewers to consider that the power of God enough to help them in any situation regardless of its gravity, taking into account Marcus' attitude in the face of an enemy that was visibly much powerful than him.

Even with the fact that she is hesitant about being a part of the cult, Nell appears to be willing to perform acts that would benefit the sect as a whole. Her tendency to put Marcus on a wrong...

Her relationship with her brother is tensed, as it is obvious that he wants to exploit her.
The fact that the film was edited by the sect means that it has focused on providing viewers with a story that would favor the cult. Taking this into account, it would appear that viewers have involuntarily supported the sect by watching the motion picture.

Nell is not faking her possession in the barn, as it would be physically impossible for someone to turn his or her neck the way that she does. The fact that she proceeds with breaking her fingers further contributes to the belief that she is actually possessed while she is being exorcised in the barn.