However, since this man was a scientist, Father Gary Thomas wasn’t surprised that he was a skeptic when it came to supernatural evil. That was fine, since one of the first things the priest learned in Rome while training to be an exorcist was to remain as skeptical as possible, as long as possible. Still, there were troubling facts in the man’s story — such as an episode when a counselor urged him to channel spirits.

Finally, the priest turned to “De Exorcismis et Supplicationibus Quibusdam (Of Exorcisms and Certain Supplications),” the Vatican document released in 1998 that contains a modernized exorcism rite. He has been fighting this man’s demons ever since.

“When we started, he told me, ‘Wait! Can’t you just take this thing right out of me?’ But that’s rarely how things work,” said Thomas, the official exorcist in the Diocese of San Jose in northern California. “It’s hard to get people to understand that no two exorcisms are the same. Reality isn’t like the movies.”

The subject of demonic possession remains controversial, as illustrated by the media storm that greeted the revised exorcism rite, which was required by a Vatican II mandate three decades earlier. Later, the Vatican announced that Pope John Paul II had personally performed three exorcisms during his pontificate.

While the new rite warned exorcists not to confuse diabolic possession with mental illness, it also affirmed ancient teachings about the reality of spiritual warfare, as illustrated by biblical accounts of Jesus performing exorcisms.

Truth is, stressed Thomas, the events of Holy Week — especially Good Friday, Holy Saturday and Easter — make no sense without real demons, real temptations and a real hell. But many Catholics disagree.

“There are plenty of bishops and priests who simply do not believe in Satan and demons and they have told me so,” he said. “That makes a difference. What most people do not realize is that bishops are like independent contractors and they can do whatever they damn well want to do. … That’s why we don’t have many exorcists in America.”

At the request of his own bishop, Thomas took a Vatican-approved approved course on demonic possession while living at the North American College in Rome in late 2005 and early 2006. As part of his studies, the second-career priest — who worked in a mortuary before seeking ordination — participated in more than 80 exorcisms with a senior Italian exorcist. These experiences form the heart of “The Rite: The Making of a Modern Exorcist,” a new book written by American journalist Matt Baglio.

For the past three years, the 55-year-old priest has quietly been using the techniques he learned in Rome. He said that his teachers, from the beginning, emphasized that an exorcist must strive to remain the “ultimate skeptic,” pursuing every pastoral option before turning to the exorcism rite as a last resort.

Modern exorcists are urged to work with psychiatrists, psychologists and physicians while evaluating those who are suffering. They also test to see if spiritual health can be restored through confession, healing rites and frequent participation in Mass. However, Thomas noted that these contacts with “holy things” occasionally trigger open displays of demonic powers.

It’s one thing to hear the voice of a demon on a recording or to read pages of blasphemies in transcripts. Face-to-face encounters are another matter.

The classic signs of possession have been established for ages. The possessed may exhibit superhuman strength, describe private events in the life of an exorcist or possess the ability to speak languages — such as Latin — they have never studied. They often suffer bizarre physical reactions to contact with holy water, crosses or icons.

Most people seeking exorcisms are simply physically sick, mentally ill or emotionally distressed. Some may try to fake “Hollywood-esque symptoms” in order to draw sympathy or attention.

“You may see case after case in which there are other explanations for what these people are suffering,” stressed Thomas. “But then, every now and then, you see things that let you know that you are dealing with the real thing. That’s when you know that sin is real, hell is real and Satan is real. That’s when you learn what the cross and the resurrection are all about.”

He didn’t see heads spin 360 degrees or volcanoes of pea-soup vomit. He was, in the end, convinced that demons are real. The results went into “The Exorcism of Emily Rose,” a chilling movie that Derrickson hopes will make believers think twice about what they believe and doubters have doubts about their doubts.

“The research phase was horrible,” he said, during press events preceding the Sept. 9 release. “I am glad that I know so much about it. That’s good knowledge to have. As a writer, it certainly is. I also feel that for me, as a Christian, it is good to have that knowledge. But I will never do that again.”

The movie was inspired by the story of Anneliese Michel, a German college student and devout Catholic who died during exorcism rites in 1976. Doctors said her seizures and visions were caused by epilepsy. Her family was convinced otherwise and their bishop agreed to allow a series of exorcism rites.

The ordeal eventually took her life. State officials prosecuted the parents and their priests for criminal negligence, leading to a trial that divided skeptics and believers — then and now.

Derrickson and co-writer Paul Harris Boardman moved this story to the American heartland, changing scores of details. The result wraps a horror movie inside a courtroom drama, with Emily’s story told in flashbacks. The big question: Is this a story of fatal abuse caused by superstition or an inspiring account of a battle with evil incarnate?

After weeks or terror, Emily writes a letter in which she describes a heavenly vision. In it, the Virgin Mary tells her that she can die peacefully or struggle on, enduring more pain but proving that demonic possession is real. On the witness stand, the family’s priest reads this letter and emphasizes this passage: “People say that God is dead. But how can they think that if I show them the devil?”

The movie is light on special effects and heavy on scenes that blur — but do not erase — the lines between faith and science, the natural and the supernatural. “The Exorcism of Emily Rose” is not a film for moviegoers who avoid the sound of creaky wooden floor planks, the scratch of fingernails on plaster walls, the howling of hellish voices in ancient tongues or the crunch of insects between human teeth. Is this insanity or spiritual warfare?

The timing was good for a movie built on spiritual questions, admitted Derrickson. A studio executive read the script and gave it a green light days after the release of Mel Gibson’s “The Passion of the Christ.”

The key, said actress Laura Linney, is that the movie doesn’t tell “people what to think or to believe.” Instead, it shows how people with different beliefs view mysterious events in different ways. The cast and crew included people with a variety of religious beliefs, as well.

Linney plays a doubter who defends the priest. The prosecutor is portrayed as a progressive Christian, a Bible-reading modernist who is repulsed by this encounter with what he considers an ancient, irrational and dangerous form of faith. Similar conflicts are dividing many religious groups today.

The goal, said Linney, was to open up “one of the big mysteries: Where does evil come from? Is it stuff in our brains or is it something outside of ourselves? Some people have very strong opinions about it, one way or the other.” Hopefully, this film “will cause both sides to re-evaluate and to listen to the other side,” she said.

In the end, Derrickson said he hopes moviegoers will dare to ask tough questions about good and evil, God and Satan.

“Right now, there is plenty of amorphous belief out there about God,” he said. “Lots of people are saying, ‘God is within us. God is a force. God is everything. God is everywhere.’ … They don’t really believe in a God who makes demands, who judges, does things that make us uncomfortable. They’re vague about evil, too.

“What we tried to do was make an entertaining movie that scared people. But I also wanted people to stop and think about all of that.”