..."The use of exorcism in what is perceived as demon possession has long been criticized by the mental health community, which cautions that ignoring mental illness can be detrimental to the patient. The Vatican upgraded its rules on exorcism in 1999, making it mandatory to consult mental health care experts when dealing with a person exhibiting signs of what had been traditionally assumed to be devil possession.
Vatican exorcist Father Cipriano de Meo argues that it is fairly easy for a seasoned exorcist to spot the difference. He says that a skilled exorcist will notice that during a prayer session with the potentially possessed, the ‘adversary’ or devil that is present will let the exorcist know he’s there.
“A possessed person has various general attitudes towards an exorcist, who is seen by the Adversary as an enemy ready to fight him,” he told Catholic news service ACI. “There's no lack of frightening facial expressions, threatening words or gestures and other things, but especially blasphemies against God and Our Lady.”
The Catholic Church teaching makes the distinction. “Exorcism is directed at the expulsion of demons or to the liberation from demonic possession through the spiritual authority which Jesus entrusted to his Church,” according to the Catholic Catechism teaching on the topic. “Illness, especially psychological illness, is a very different matter; treating this is the concern of medical science. Therefore, before an exorcism is performed, it is important to ascertain that one is dealing with the presence of the Evil One, and not an illness.”
Still, the exorcism rite is hardly like it is depicted in pop culture. The Catholic Church Book of Rites says that the exorcist must first go to confession to clear his own soul, thereby not offering any competition in the way of an unclean soul for the devil during the ritual. Then, wearing a purple stole over a simple tunic, he sprinkles himself and the possessed person with holy water and recites a list of prayers including the Litany of the Saints and others before reciting the final devil-be-gone prayer that starts with, “I cast you out, unclean spirit, along with every Satanic power of the enemy, every specter from hell, and all your fell companions…”
The rite can be repeated as often as is necessary and the Church advises supplemental prayer to keep the devil away for good. "