How can you determine whether a person is possessed by an evil spirit or is simply suffering from a mentally illness?

The answer may not be apparent to ordinary people since a possessed person and one who is mentally ill may exhibit similar physical reaction.

However, Father Cipriano de Meo, a long-time exorcist and priest in Rome, said a well-trained exorcist should know the difference.

Speaking to CNA's Italian agency ACI Stampa, De Meo said in many alleged cases of possession, the victims turned out to be just struggling with some form of mental illness.

An exorcist since 1952, De Meo said the key to telling the difference is through discernment in prayer on the part of the exorcist and the possessed and in the potentially possessed person's reaction to the exorcist himself and the prayers being said.

The exorcist will typically say "(a) prolonged prayer to the point where if the Adversary is present, there's a reaction," he said.

"A possessed person has various general attitudes towards an exorcist, who is seen by the Adversary as an enemy ready to fight him," he added.

De Meo said one telling indication that a person is possessed is the latter's response to the exorcist's prayer.

"There's no lack of frightening facial expressions, threatening words or gestures and other things," he said, "but especially blasphemies against God and Our Lady."

The veteran exorcist priest said the best defence against demonic possession is a simple and sacramental life of prayer.

"It's absolutely fundamental to get rid of sin and live in the grace of God," he said.

The Catechism of the Catholic Church (CCC) offers detailed guidance on how to avoid demonic possession.

It calls on the faithful to avoid "consulting horoscopes, astrology, palm reading, interpretation of omens and lots, the phenomena of clairvoyance, and recourse to mediums," saying they "all conceal a desire for power over time, history, and, in the last analysis, other human beings, as well as a wish to conciliate hidden powers."

"They contradict the honor, respect, and loving fear that we owe to God alone," it says.

De Meo said the exorcists need to remain humble and to remember that their power comes from Christ.

"Regarding spiritual preparation, humility and the conviction that we exorcists aren't the ones who are going to cast out the demon that's fighting Christ. We're called to fight on behalf of Christ," he said.