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 (film)
Both priests witness Regan perform a series of bizarre, vulgar acts. They attempt to exorcise the demon, but the stubborn entity toys with them, especially Karras. Karras shows weakness and is dismissed by Merrin, who attempts the exorcism alone. Karras enters the room later and discovers Merrin has died of a heart attack. After failing to revive Merrin, the enraged Karras confronts the mocking, laughing spirit, and wrestles Regan's body to the ground. At Karras's invitation, it leaves Regan's body and possesses Karras. In a moment of self-sacrifice, the priest throws himself out of the window before being compelled to harm Regan. He is mortally injured. Father Dyer, an old friend of Karras, happens upon the scene and administers the last rites to his friend.