The Exorcist (1973)

When a young girl is possessed by a demon, her mother will seek all kinds of medical help, until she discovers that she must turn to the church. A priest, with insecurities about his own faith, and an experienced exorcist come to their aid. Besides being a landmark horror film that has stood the test of time, the direction of Friedkin is masterfull in reinforcing the theme of good vs. evil.