Exorcism
In Catholic Christianity, exorcisms are performed in the name of Jesus Christ. A distinction is made between a formal exorcism, which can only be conducted by a priest during a baptism or with the permission of a bishop, and "prayers of deliverance" which can be said by anyone.
The Catholic rite for a formal exorcism, called a "Major Exorcism", is given in Section 11 of the Rituale Romanum. The Ritual lists guidelines for conducting an exorcism, and for determining when a formal exorcism is required. Priests are instructed to carefully determine that the nature of the affliction is not actually a psychological or physical illness before proceeding.
The Last Exorcism
Filmmakers Iris and Daniel follow Cotton Marcus, a reverend living in Baton Rouge, Louisiana, who seeks to delegitimize exorcisms. Marcus, who lost his faith after the birth of his ill son, is accustomed to performing fake exorcisms on "possessed" individuals. He accepts an exorcism request sent by farmer Louis Sweetzer, who claims his livestock are being slaughtered by his daughter Nell; Louis suspects that Nell is possessed by Satan.