In a nutshell. Girl get possessed by a demon that she allegedly introduced by using a Ouija board. They consult a number of doctors who after administering test conclude there is nothing either physically or emotionally wrong with her and suggested an exorcist be consulted. The first priest, Damon is losing his faith and skeptical about the nature of his religion. The second priest is a pious and experienced and has a physical premonition that there is going to be a confrontation between him and the demon while at an archeological dig. Ultimately the first priest invites the demon to leave the girl and posses him, he dies and discovers that there is a heaven and hell.
There is a cult in the village that was organized by the local reverend. It has members who include a lot of kids and Caleb is also among them. There is an arrangement for Nell to get impregnated with the incarnation of a demon. Check out the additional summary from http://www.imdb.com/title/tt1320244/plotsummary.