(CNN) A small group of nuns and priests met the woman in the chapel of a house one June evening. Though it was warm outside, a palpable chill settled over the room.

As the priests began to pray, the woman slipped into a trance -- and then snapped to life. She spoke in multiple voices: One was deep, guttural and masculine; another was high-pitched; a third spouted only Latin. When someone secretly sprinkled ordinary water on her, she didn't react. But when holy water was used, she screamed in pain.

"Leave her alone, you f***ing priests," the guttural voice shouted. "Stop, you whores. ... You'll be sorry."

You've probably seen this before: a soul corrupted by Satan, a priest waving a crucifix at a snarling woman. Movies and books have mimicked exorcisms so often, they've become clichés.

The 1973 film "The Exorcist" shaped how many see demonic possession.

But this was an actual exorcism -- and included a character not normally seen in the traditional drive-out-the-devil script.

Dr. Richard Gallagher is an Ivy League-educated, board-certified psychiatrist who teaches at Columbia University and New York Medical College. He was part of the team that tried to help the woman.

Fighting Satan's minions wasn't part of Gallagher's career plan while he was studying medicine at Yale. He knew about biblical accounts of demonic possession but thought they were an ancient culture's attempt to grapple with mental disorders like epilepsy. He proudly calls himself a "man of science."

Yet today, Gallagher has become something else: the go-to guy for a sprawling network of exorcists in the United States. He says demonic possession is real. He's seen the evidence: victims suddenly speaking perfect Latin; sacred objects flying off shelves; people displaying "hidden knowledge" or secrets about people that they could not have possibly have known.

"There was one woman who was like 90 pounds soaking wet. She threw a Lutheran deacon who was about 200 pounds across the room," he says. "That's not psychiatry. That's beyond psychiatry."

Gallagher calls himself a "consultant" on demonic possessions. For the past 25 years, he has helped clergy distinguish between mental illness and what he calls "the real thing." He estimates that he's seen more cases of possession than any other physician in the world.

There was one woman who was like 90 pounds soaking wet. She threw a Lutheran deacon who was about 200 pounds across the room. That's not psychiatry. That's beyond psychiatry. Dr. Richard Gallagher, psychiatrist, professor and demonic possession "consultant"

"Whenever I need help, I call on him," says the Rev. Gary Thomas, one of the most famous exorcists in the United States. The movie "The Rite" was based on Thomas' work.

"He's so respected in the field," Thomas says. "He's not like most therapists, who are either atheists or agnostics."

Gallagher is a big man -- 6-foot-5 -- who once played semipro basketball in Europe. He has a gruff, no-nonsense demeanor. When he talks about possession, it sounds as if he's describing the growth of algae; his tone is dry, clinical, matter-of-fact.

Possession, he says, is rare -- but real.

"I spend more time convincing people that they're not possessed than they are," he wrote in an essay for The Washington Post.

Some critics, though, say Gallagher has become possessed by his own delusions. They say all he's witnessed are cheap parlor tricks by people who might need therapy but certainly not exorcism. And, they argue, there's no empirical evidence that proves possession is real.

Still, one of the biggest mysteries about Gallagher's work isn't what he's seen. It's how he's evolved.

How does a "man of science" get pulled into the world of demonic possession?

His short answer: He met a queen of Satan.

A 'creepy' encounter with evil

She was a middle-age woman who wore flowing dark clothes and black eye shadow. She could be charming and engaging. She was also part of a satanic cult.

She called herself the queen of the cult, but Gallagher would refer to her as "Julia," the pseudonym he gave her.

The woman had approached her local priest, convinced she was being attacked by a demon. The priest referred her to an exorcist, who reached out to Gallagher for a mental health evaluation.

Why, though, would a devil worshipper want to be free of the devil?

"She was conflicted," Gallagher says. "There was a part of her that wanted to be relieved of the possession."

She ended up relieving Gallagher of his doubts. It was one of the first cases he took, and it changed him. Gallagher helped assemble an exorcism team that met Julia in the chapel of a house.

Objects would fly off shelves around her. She somehow knew personal details about Gallagher's life: how his mother had died of ovarian cancer; the fact that two cats in his house went berserk fighting each other the night before one of her sessions.

Julia found a way to reach him even when she wasn't with him, he says.

He was talking on the phone with Julia's priest one night, he says, when both men heard one of the demonic voices that came from Julia during her trances -- even though she was nowhere near a phone and thousands of miles away.

He says he was never afraid.

"It's creepy," he says. "But I believe I'm on the winning side."

How a scientist believes in demons

He also insists that he's on the side of science.

He says he's a stickler for the scientific method, that it teaches people to follow the facts wherever they may lead.

Growing up in a large Irish Catholic family in Long Island, he didn't think much about stories of possession. But when he kept seeing cases like Julia's as a professional, he says, his views had to evolve.

Some priests say those who dabble in the occult are opening doorways to the demonic.

"I don't believe in this stuff because I'm Catholic," he says. "I try to follow the evidence."

Being Catholic, though, may help.

Gallagher grew up in a home where faith was taken seriously. His younger brother, Mark, says Gallagher was an academic prodigy with a photographic memory who wanted to use his faith to help people.

"We had a sensational childhood," Mark Gallagher says. "My mother and father were great about always helping neighbors or relatives out." Their mother was a homemaker, and their father was a lawyer who'd fought in World War II. "My father used to walk us proudly into church. He taught us to give back."

Gallagher's two ways of giving back -- helping the mentally ill as well as the possessed -- may seem at odds. But not necessarily for those in the Catholic Church.

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One of Gallagher's favorite sources of inspiration is Pope John Paul II's encyclical "Fides et Ratio" ("On Faith and Reason"). The Pope writes that "there can never be a true divergence between faith and reason, since the same God who reveals the mysteries and bestows the gift of faith has also placed in the human spirit the light of reason."

The church's emphasis on faith and reason can even been seen in the birth of its exorcism ritual.

The Rite of Exorcism was first published in 1614 by Pope Paul V to quell a trend of laypeople and priests hastily performing exorcisms on people they presumed were possessed, such as victims of the bubonic plague, says the Rev. Mike Driscoll, author of "Demons, Deliverance, Discernment: Separating Fact from Fiction about the Spirit World."

"A line (in the rite) said that the exorcist should be careful to distinguish between demon possession and melancholy, which was a catchall for mental illness," Driscoll says. "The church knew back then that there were mental problems. It said the exorcist should not have anything to do with medicine. Leave that to the doctors."

Doctors, perhaps, like Gallagher.

Gallagher says the concept of possession by spirit isn't limited to Catholicism. Muslim, Jewish and other Christian traditions regard possession by spirits -- holy or benign -- as possible.

"This is not quite as esoteric as some people make it out to be," Gallagher says. "I know quite a few psychiatrists and mental health professionals who believe in this stuff."

Dr. Mark Albanese is among them. A friend of Gallagher's, Albanese studied medicine at Cornell and has been practicing psychiatry for decades. In a letter to the New Oxford Review, a Catholic magazine, he defended Gallagher's belief in possession.

He also says there is a growing belief among health professionals that a patient's spiritual dimension should be accounted for in treatment, whether their provider agrees with those beliefs or not. Some psychiatrists have even talked of adding a "trance and possession disorder" diagnosis to the DSM, the premier diagnostic manual of disorders used by mental health professionals in the US.

There's still so much about the human mind that psychiatrists don't know, Albanese says. Doctors used to be widely skeptical of people who claimed to suffer from multiple personalities, but now it's a legitimate disorder (dissociative identity disorder). Many are still dumbfounded by the power of placebos, a harmless pill or medical procedure that produces healing in some cases.

"There's a certain openness to experiences that are happening that are beyond what we can explain by MRI scans, neurobiology or even psychological theories," Albanese says.

Dr. Jeffrey Lieberman, a psychiatrist who specializes in schizophrenia, arrived at a similar conclusion after he had an unnerving experience with a patient.

Lieberman was asked to examine the videotape of an exorcism that he subsequently dismissed as unconvincing.

Then he met a woman who, he said, "freaked me out."

Lieberman, director of the New York State Psychiatric Institute, says he and a family therapist were asked to examine a young woman who some thought was possessed. He and his colleague tried to treat the woman for several months but gave up because they had no success.

The film "The Rite" is based on the life of the Rev. Gary Thomas, one of the leading exorcists in the US.

Something happened during the treatment, though, that he still can't explain. After sessions with the woman, he says, he'd go home in the evenings, and the lights in his house would go off by themselves, photographs and artwork would fall or slide off shelves, and he'd experience a piercing headache.

When he mentioned to this to his colleague one day, her response stunned him: She'd been having the exact same experiences.

"I had to sort of admit that I didn't really know what was going on," Lieberman says. "Because of the bizarre things that occurred, I wouldn't say that (demonic possession) is impossible or categorically rule it out ... although I have very limited empirical evidence to verify its existence."

The tragic case of the real 'Emily Rose'

If you want to know why so many scientists and doctors like Lieberman are cautious about legitimizing demonic possession, consider one name: Anneliese Michel.

Michel was a victim in one of the most notorious cases of contemporary exorcism. If you have the stomach for it, go online and listen to audiotapes and watch videos of her exorcisms. The images and sounds will burn themselves into your brain. It sounds like somebody dropped a microphone into hell.

Michel was a German Catholic woman who died of starvation in 1976 after 67 exorcisms over a period of nine months. She was diagnosed with epilepsy but believed she was possessed. So did her devout Roman Catholic parents. She reportedly displayed some of the classic signs of possession: abnormal strength, aversion to sacred objects, speaking different languages.

But authorities later determined that it was Michel's parents and two priests who were responsible for her death. German authorities put them on trial for murder, and they were found guilty of negligent homicide. The 2005 film "The Exorcism of Emily Rose" was based on Michel's ordeal and the subsequent trial.

The worst thing you can do to a patient who is delusional is to confirm their delusions. Telling a patient who is struggling that maybe they're possessed by a demon is the worst thing you can do. Steven Novella, a neurologist and professor at Yale School of Medicine

One of the leading skeptics of exorcism -- and one of Gallagher's chief critics -- is Steven Novella, a neurologist and professor at Yale School of Medicine.

He wrote a lengthy blog post dissecting Gallagher's experience with Julia, the satanic priestess. It could be read as a takedown of exorcisms everywhere.

He says Julia probably performed a "cold reading" on Gallagher. It's an old trick of fortune tellers and mediums in which they use vague, probing statements to make canny guesses about someone. (Fortune teller: "I see a recent tragedy in your family." Client: "You mean my sister who got hurt in a car accident? How did you know?")

Or take the case of a person speaking an unfamiliar language like Latin during a possession.

"A patient might memorize Latin phrases to throw out during one of their possessions," Novella wrote. "Were they having a conversation in Latin? Did they understand Latin spoken to them? Or did they just speak Latin?"

Novella says it's noteworthy that no one has filmed any paranormal event such as levitation or sacred objects flying across the room during an exorcism. He's seen exorcism tapes posted online and in documentaries and says they're not scary.

"They're boring," he says. "Nothing exciting happens. The most you get is some really bad play-acting by the person who is being exorcised."

In an interview, Novella went further and criticized any therapist who believes his patient's delusions.

"The worst thing you can do to a patient who is delusional is to confirm their delusions," says Novella, who founded the New England Skeptical Society.

"The primary goal of therapy is to reorient them to reality. Telling a patient who is struggling that maybe they're possessed by a demon is the worst thing you can do. It's only distracting them from addressing what the real problem is."

The 2005 horror film "The Exorcism of Emily Rose" was loosely based on the death of Anneliese Michel.

Driscoll, the Catholic priest who wrote a book about possession, is not a skeptic like Novella. Still, he says, it's not unusual for people on drugs or during psychotic episodes to display abnormal strength.

"I have seen it take four grown guys to hold one small woman down," says Driscoll, a chaplain at St. Elizabeth Medical Center in Ottawa, Illinois. "When a person has no fear and is not in their right mind and they don't care about hurting themselves or hurting others, you can see heartbreaking things."

That doesn't mean he thinks possession isn't real. He says the New Testament is full of accounts of Jesus confronting demons.

"Do I still believe it happens? Yes, I do," he says. "It happened then. I don't know why it would be totally eradicated now."

Gallagher agrees and has answers for skeptics like Novella.

He says demons won't submit to lab studies or allow themselves to be easily recorded by video equipment. They want to sow doubt, not confirm their existence, he says. Nor will the church compromise the privacy of a person suffering from possession just to provide film to skeptics.

Gallagher says he sees his work with the possessed as an extension of his responsibilities as a doctor.

In a passage from a book he is working on about demonic possession in America, he says that it is the duty of a physician to help people in great distress "without concern whether they have debatable or controversial conditions."

Gallagher isn't the first psychiatrist to feel such duty. Dr. M. Scott Peck, the late author of "The Road Less Traveled," conducted two exorcisms himself -- something Gallagher considers unwise and dangerous for any psychiatrist.

"I didn't go volunteering for this," he says. "I went into this because different people over the last few decades realized that I was open to this sort of thing. The referrals are almost invariably from priests. It's not like someone is walking into my office and I say, 'You must be possessed.' "

What happened to Satan's queen

He may not have asked to join the "hidden" world of exorcism, but he is an integral part of that community today. He's been featured in stories and documentaries about exorcism and is on the governing board of the Rome-based International Association of Exorcists.

"It's deepened my faith," he says of the exorcisms he's witnessed. "It didn't radically change it, but it validated my faith."

He says he's received thanks from many people he's helped over the years. Some wept, grateful to him for not dismissing them as delusional. As for letting a journalist talk to any of these people, Gallagher says he zealously guards their privacy.

Belief in possession exists in many religious traditions. Here, a man enters a state of possession during an African voodoo ceremony.

Julia, though, gave him permission to tell her story. But it didn't have a happy ending.

He and a team of exorcists continued to see her, but eventually, she called a halt to the sessions. She was too ambivalent. She relished some of the abilities she displayed during her trances. She was "playing both sides."

"Exorcism is not some kind of magical incantation," Gallagher says. "Normally, a person has to make their own sincere spiritual efforts, too."

About a year after she dropped out, Gallagher says, he heard Julia's voice on the phone again. This time, she had called to tell him she was dying of cancer.

Gallagher says he offered to try to help her with a team of priests while she was still physically able, but her response was terse:

"Well, I'll give it some thought."

He says he never heard from her again.

Inevitably, there will be others. His phone will ring. A priest will tell him a story. A team of clergy and nuns will be summoned. And the man of science will enter the hidden world of exorcism again.

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The critics, the souls that aren't saved, the creepy encounters -- they don't seem to deter him.

"Truly informed exorcists don't tend to get discouraged," he says, "because they know it is our Lord who delivers the person, not themselves."

Is Gallagher doing God's work, or does he need deliverance from his own delusions?

Perhaps only God -- and Satan -- knows for sure.