Anthony Hopkins portrays Father Lucas Trevant, a veteran exorcist, in the new film The Rite, which opens Friday. The story follows Trevant as he teaches a younger priest how to tell if a person is possessed by a demon, and what to do if that is indeed the case.

Many religions claim that humans can be possessed by demonic spirits, and offer remedies to address this inconvenience. The Bible recounts six instances of Jesus casting out demons, while voodoo and Catholicism proscribe elaborate rituals and cleansings to remove spiritual stains.

The Vatican (which, as the film accurately notes, offers courses on exorcisms) accepts only a small percentage of demonic possessions as “authentic,” which of course suggests that there are a lot of unauthentic cases of possession out there. The Vatican issued official guidelines on exorcism in 1614, and revised them in 1999.

How do you know if you or a loved one is possessed?

Demonic Possession, Reincarnation, and Xenoglossia

According to the U.S. Conference of Catholic Bishops, if you have an aversion to holy water or crosses, you might be possessed. If you get or a rash when you enter a church, you just might need an exorcist (if you just get bored in church, you’re normal). If you exhibit superhuman strength, you may be drawing your powers from the demonic dark realms.

If you suddenly are able to speak in strange languages (such as Aramaic or Esperanto, in a phenomenon called xenoglossia), that too could be a sign that a demonic polyglot has got ahold of you. This is only a partial list, and might include anything from spitting to cursing to excessive masturbation (whatever “excessive” is for you).

In his rather credulous book "Possession and Exorcism" (1988, New Frontiers Center) Hans Naegli-Osjord writes that “Demonic possession is expressed in word and deed, and occasionally in paranormal events blaspheming and obscene insults of persons present, destructive fury toward sacred objects and all kinds of utensils, and often through self-destructive acts. The possessed person beats his head against the wall, for example, and frequently inflicts painful cuts on himself.”

Dipping into somewhat more dubious territory, Naegli-Osjord also includes “poltergeist phenomena of all kinds, … smells of putrefaction and the stench of sulphur. In drastic cases… feathers, iron fragments, and smelly fluids emerge from the surfaces of the body or are vomited.” Pretty heady stuff.

If your possession is authenticated by someone claiming to be a priest or an exorcist, things might get interesting. Exorcisms take a wide variety of forms, including whipping and torture. During the Catholic exorcism ritual, you will be sprinkled with holy water while the exorcist reads a few passages from the Bible.

The priest may show you his cross, which you may be asked to inspect or even kiss. He will say “God please take care of this person and dismiss the demon,” or some variation thereof, though that doesn’t always work and more extreme measures may be called for.

If you’re smart (or have any doubt at all that you were in fact possessed) this is a good time to assure the exorcist that the demon is gone, everything will be fine, and he doesn’t really need to go any further. If the exorcist believes you, he will stop and everyone can calm down and go home; if instead he believes that the demon inside you is trying to trick him, you may be in for a very long night.

Or, in fact, you may be killed. There are numerous cases of people being murdered by others who suspected them of being possessed by demons.

It's very unlikely that you'll experience anything remotely resembling stuff in the movies. Author Michael Cuneo, who participated in over 50 exorcisms while researching his book "American Exorcism: Expelling Demons in the Land of Plenty" (2002, Broadway Books) saw nothing remotely resembling the remarkable events depicted in The Exorcist.

Photo: Anthony Hopkins in "The Rite." credit: New Line Cinema — Warner Bros