The Exorcist is a 1973 American supernatural horror-drama film directed by William Friedkin, adapted by William Peter Blatty from his 1971 novel of the same name. The book, inspired by the 1949 exorcism of Roland Doe, deals with the demonic possession of a 12-year-old girl and her mother's attempts to win back her child through an exorcism conducted by two priests. The adaption is relatively faithful to the book, which itself has been commercially successful (hitting the New York Times bestseller list). Wikipedia
Chris MacNeil: You show me Regan's double, same face, same voice, everything. And I'd know it wasn't Regan. I'd know in my gut. And I'm telling you that 'thing' upstairs isn't my daughter. Now, I want you to tell me that you know for a fact that there's nothing wrong with my daughter, except in her mind. You tell me for a fact that an exorcism wouldn't do any good. You tell me that!