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 Exorcist (novel)
After several unsuccessful psychiatric and medical treatments, Regan's mother, an atheist, turns to a local Jesuit priest for help as Regan's personality becomes increasingly disturbed. Father Damien Karras, who is currently going through a crisis of faith coupled with the loss of his mother, agrees to see Regan as a psychiatrist, but initially resists the notion that it is an actual demonic possession. After a few meetings with the child, now completely inhabited by a diabolical personality, he turns to the local bishop for permission to perform an exorcism on the child.