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.
Exorcism in Christianity
Solemn exorcisms, according to the Canon law of the church, can be exercised only by an ordained priest (or higher prelate), with the express permission of the local bishop, and only after a careful medical examination to exclude the possibility of mental illness, and in the ritual people cannot in any circumstance be harmed. The "Catholic Encyclopedia" (1908) enjoined: "Superstition ought not to be confounded with religion, however much their history may be interwoven, nor magic, however white it may be, with a legitimate religious rite." Things listed in the Roman Ritual as being indicators of possible demonic possession include: speaking foreign or ancient languages of which the possessed has no prior knowledge; supernatural abilities and strength; knowledge of hidden or remote things which the possessed has no way of knowing; an aversion to anything holy; and profuse blasphemy and/or sacrilege.