The horror flick is based on "The Making of a Modern Exorcist" by Matt Baglio -- and yes, that book is non-fiction. The Vatican really does sponsor university-level exorcism classes, and an American priest really did attend those classes at the behest of his bishop. No, he didn't encounter a possessed Anthony Hopkins in real life ... but Father Gary Thomas
does say that about 15 percent of his work week is taken up by exorcism-related duties. In fact, some Catholic publications objected to the film being classified as horror -- and said it should have been filmed as a straight-up docu-drama.