Exorcism
In Christianity, exorcism is the practice of casting out demons. In Christian practice the person performing the exorcism, known as an exorcist, is often a member of the Christian Church, or an individual thought to be graced with special powers or skills. The exorcist may use prayers and religious material, such as set formulas, gestures, symbols, icons, amulets, etc. The exorcist often invokes God, Jesus and/or several different angels and archangels to intervene with the exorcism. Protestant Christian exorcists most commonly believe the authority given to them by the Father, Son, and Holy Spirit (the Trinity) is the source of their ability to cast out demons.
The Exorcist (novel)
Recent investigative research by freelance journalist Mark Opsasnick indicates that Blatty's novel was based on an actual 1949 exorcism of a young boy from Cottage City, Maryland, whom Opsasnick refers to using the pseudonyms Robbie Mannheim and Roland Doe. The boy was sent to his relative's home on Roanoke Drive in St. Louis where most of the exorcism took place.