The exorcism conference was closed-door. The October conference, in Baltimore, Maryland, preceding a conference of US Catholic bishops, was attended by 56 bishops and 66 priests. The bishops were the novelty. There is a small annual conference on exorcism, but only a few priests attend. The conference focused on the details of the rite of exorcism, and "the reality of demonic manifestations in our society", a participant said.

While all Catholic priests can theoretically perform exorcism, very few American priests are trained to do so. The conference, intended to remedy the shortfall, was organised by Bishop Thomas Paprocki, chair of the Bishops' committee on canonical affairs and church governance. "I thought it went well," he said. "We had lots of bishops, and lots of priests."

While no one seems to have hard data on the number of US exorcisms, Paprocki said there's increasing demand for them. "There were only a small number of priests who had any real expertise, and they were getting calls from all over the country." So many attended the conference, he said – "They must have requests."

There are 68 million Roman Catholics in the United States. Vague estimates of the number of exorcists available to them range from five or six to 14.

Paprocki hasn't performed exorcisms, so he referred me to Reverend Gary Thomas. Thomas is a parish priest in Saratoga, California, a wealthy suburb near Silicon Valley. I live 50 miles away in San Francisco, so he's the priest I would be sent to if I were a practicing Catholic, and possessed. (He is the subject of The Rite, a book and a forthcoming movie starring Anthony Hopkins.)

"There is a need for more exorcists," Thomas agreed. The demand comes from parishioners, who tell priests they or a family member need exorcism. Thomas ascribes increased demand to increased "activity of paganism and idolatrous practices among Catholics," and to people confusing mental disease with possession.

On the other hand, Reverend Richard Vega, president of the National Federation of Priests' Councils, told the New York Times he hadn't heard of any requests for exorcism. Vega speculated that immigrant Catholics might be more familiar with exorcism, and more inclined to ask their priests for it. "Canon law requires that every diocese have an exorcist and most do not," Thomas told me, somewhat grimly. "It's a result of the post-Vatican II church."

In 2005, Pope John Paul II wrote a letter requiring every US bishop to designate an exorcist, Thomas tells me. His bishop asked him to take on the responsibility (after the first candidate declined). It was nothing Thomas aspired to. "I didn't sign up for this." While on sabbatical in Rome he took a course and studied with one of Rome's exorcists.

Exorcism begins with discernment – determining whether the person is possessed, or just thinks they are (or someone else thinks they are). "We don't do them on demand," says Thomas. I was interested by the possibility of people being brought in by others. "The person who is possessed may not even realise it. It's more frequent that someone would bring a person in," Paprocki said. (This is my fear. What if my mother decides I'm possessed? My neighbour? My editor?)

A team does discernment. "I have a physician, a clinical psychologist and a psychiatrist, all of whom are practicing Catholics," says Thomas. "Out of a hundred I exorcise five." He says the exorcist is the ultimate skeptic. "I never assume when someone says 'I need an exorcism' that they do."

Possession, Paprocki says, is an extraordinary situation. "We're talking about a relationship between a person and a demon. Perhaps the relationship goes bad. Perhaps they see the darkness and want out. This happens in relationships, and it's the same with the demon.

"Some demons are very strong," he said. "So it needs repeated prayer and fasting and penance." People breaking up relationships don't always make a clean break. "The relationship didn't develop overnight and is not broken overnight." Thus exorcisms may require repetition, something I didn't grasp till I talked to the man who does them.

I asked Thomas how many exorcisms he's performed. "About 40 over five years. Involving five people." Two gave up, saying it was taking too long. Exorcism is "a set of prayers that may break a relationship between a demon and a human being." Thomas declined to call the task rewarding. Meaningful, perhaps, but "the victories are small in number, and it takes a lot of time. It's very arduous work".

Both bishop and priest regularly reassure people about demonic possession. "Possession is not an involuntary thing. It's not contagious. The person themselves has to open the door," Paprocki said. To those who come to Thomas asking: "How do I protect myself from being inhabited by a demon?" he says: "If you have a prayer life, if you have a sacrament life, if you have a faith life – you have nothing to fear."

To be possessed, "you have to invite a demon in. Demons are always looking for broken or nonexistent relationships between human beings. I'll give you an example. Eighty percent of the people who come to see me have been sexually abused by a family member [or other person]," Thomas said. "Sexual abuse is a doorway for a demon. It doesn't mean that they will be possessed. It does mean that they're vulnerable."

Exorcisms that hit the news are freelance efforts gone terribly wrong. Amateurs trying to banish demons suffocate, drown, or beat people to death. Training in exorcism might be viewed as a public safety matter.

An observant Catholic friend, citing inept attempts at exorcism by certain disciples in the gospels, hinted that unskilled exorcism is dangerous to the exorcist. Paprocki dismissed this. "It's not that it's dangerous, they're just ineffective," he said. "In the end Jesus is more powerful than the devil."

Thomas, the reluctant exorcist, took a grumpier view. "Yes. The exorcist gets attacked. There are emotional attacks, and psychological attacks, and physical attacks – I can't say that I've been physically attacked. My celibacy gets attacked a lot."

Paprocki and Thomas referred to demons and devils, and to devils in the plural and singular. "I use those interchangeably," Paprocki said. "Sometimes a person can be possessed by more than one devil."

"A devil is a spiritual being who has rejected God and suffers eternal torment because of that." Devils have names. "Part of the ritual is to ask the name of the demon. That gives the exorcist power over them. The devil is also the Prince of Lies, so they may lie about the name, but it's something that the priest uses in commanding the devil."

Exorcism is rare, because possession is rare. "The ordinary activity of the devil is temptation," Paprocki said. "It's a big mistake for people to think that the only activity of the devil is possession – and 'If I'm not possessed I don't need to worry about the devil.'"