I guess you can look at this horror sub-genre in two ways. The primal fear of losing control of your body and your free will is so powerful that these movies have a built in audience. However, and this is the standpoint I subscribe to, is that every demonic possession film after The Exorcist is a ale substitute. That 1973 film said pretty much all there is to say about the topic, as evidenced by other films in this sub-genre who try to put a “fresh spin” on the subject.
Case in point, this film, which focuses on a group of exorcists who are not connected with the church. In a perfect world, they would drive around in a jury-rigged hearse with a siren that is simply a loop of “The Power of Christ Compels You.” Unfortunately, this is a more somber treatment as the amateur exorcists take on the case of a woman who kill three people at her last, Church-sanctioned exorcism. Good luck!