The rite, those few priests who have performed it say, can unfold as a quiet prayer session or a show of violence.
The afflicted person may curse the cleric, speak in a voice not his or her own, even assume facial features that one priest described as "reptilian."
But in the great majority of cases in which a Catholic seeks an exorcism, church officials say, what the person really needs is help of a less dramatic nature: a doctor, a therapist or simple pastoral counseling.
Exorcism depends on an understanding of the devil as not merely a symbol of evil but as a real being, capable of acting in the world. Several recent popes, including Pope Benedict and his predecessor, Pope John Paul II, have taken steps to stress that understanding.
"Evil is not merely a lack of something, but an effective agent, a living spiritual being, perverted and perverting. A terrible reality," Pope Paul VI told a general audience in 1972.
Pope John Paul II, who is reported to have performed three exorcisms, said in 1987: "The battle against the devil … is still being fought today, because the devil is still alive and active in the world."
And shortly after he took office in 2005, Pope Benedict publicly praised a gathering of exorcists, referring to their work as an "important ministry."
Pope Benedict was head of the Vatican's Congregation for the Doctrine of the Faith in 2004 when it asked bishops throughout the world to appoint exorcists in their dioceses. It's not clear how seriously the request was taken.
But for all the high-level attention, the Rev. Thomas Reese said, exorcism is "certainly not at the core of who we are as Christians, even though it is mentioned in the Bible."
"I'm very agnostic in approaching any of this stuff," said Reese, a theologian at Georgetown University. "I've never seen a possession. I hope I never do."
While the supernatural is a part of Christianity, Reese said, he prefers the emphasis on good works in this world. He cites the Gospel of Matthew: "For I was hungry and you gave me something to eat, I was thirsty and you gave me something to drink. …"
Reese said pastors should not forget the crucial focus on guiding and mentoring their flocks.
In the final analysis, he said, "you're not going to be asked how many exorcisms you did."
With some parishes seeing an increase in claims of demonic possession in the United States, the Roman Catholic Church is training its clergy in how to respond to requests for the ancient rite. More than 100 bishops and priests attended a November workshop on the subject in Baltimore.
Bishop Thomas J. Paprocki, who organized the two-day, closed-door event at the Baltimore Marriott Waterfront, says pastors need help discerning the difference between those who need an exorcist and those who only believe they do.
The goal, he says, was to help the clergy counsel people who believe they are possessed by a demon by referring them to a physician, a therapist, or - in very rare cases, he stresses - an exorcist.
"We have only a small number of priests who have any training in this area in the United States," said Paprocki, who heads the Diocese of Springfield, Ill. "Every diocese should really have its own resources."
While no one is keeping statistics, he says anecdotal reports suggest that the phenomenon of people claiming to be possessed "seems to have increased in the last five years or so."
Clergy and theologians offer several possible explanations: the growth of the Charismatic Renewal Movement, with its traditions of mysticism and ecstatic worship; renewed interest in the occult, abetted by the Internet; the influence of television shows about the paranormal and the many cinematic descendants of "The Exorcist," the 1973 film that spiked modern interest in the subject.
The burgeoning number of Catholic immigrants from Latin America and Africa, where belief in demonic possession is common, also has been suggested by some theologians as a possible factor.
At least one opinion survey suggests that substantial numbers of Americans believe in the possibility of demonic possession. A 2001 Gallup poll found 1,012 people split in their belief in demonic possession; 41 percent believed and 41 percent did not.
Paprocki estimated that about 30 of the 195 Catholic dioceses in the United States have appointed exorcists. The U.S. Conference of Catholic Bishops does not keep statistics, a spokeswoman said, as dioceses keep their exorcists' identities confidential.
The Archdiocese of Baltimore has not appointed an exorcist, spokesman Sean Caine said. He said he could find no record of the rite ever having been performed here.
"We have four priests in the Archdiocese who have experience, and who can investigate cases of paranormal complaints or suspected demonic activity," Caine wrote in an e-mail. Of the "very few" recent cases they have investigated, he said, "none required an exorcism."
The Baltimore event was the first Paprocki knows of to be sanctioned by the bishops' conference, and the number of attendees suggests to him that interest in demonic possession is strong. Paprocki, who chairs the conference's Committee on Canonical Affairs and Church Governance, said he was "pleasantly surprised" when more than 50 priests and more than 60 bishops turned out.
While exorcism has attracted both popular interest and official attention in the decades since the reforms of the Second Vatican Council in the 1960s, some Catholic theologians despair of the very subject as an anachronism that focuses on the devil rather than Jesus.
Thomas Groome, a theologian at Boston College, called the Baltimore session "a very peculiar event."
"The reality of evil in the modern world is patently obvious," he said, but recourse to supernatural explanations seems a "throwback" - and a convenient scapegoat.
He cited comments by Pope Benedict XVI blaming demonic causes for the emergence of the clergy sex abuse scandal in Europe.
"If you say pedophile priests and abuse are the work of the devil," Groome said, "you take responsibility off of the perpetrators and those who protected them."
The Rev. Harvey Egan, also a B.C. theologian, said that while exorcism can be considered "part and parcel" of Christian faith, he worried about "all these people saying 'the devil, the devil,' when they should be saying 'Christ, God.' "
The Baltimore gathering was divided into three parts. The first explored the scriptural background on demonic possession. The New Testament includes several accounts of Jesus Christ casting out demons.
The second part, which Paprocki said took up most of the time, was devoted to the work of screening people who ask for an exorcism. Paprocki and others describe this as the sort of interview a counselor might conduct, exploring all possible natural causes for the person's experience: are they drinking or taking drugs? What is their family history? Have they been sexually abused?
"The exorcist has to be the ultimate skeptic," said the Rev. Gary Thomas, the exorcist for the Diocese of San Jose, Calif., who attended the Baltimore event. "My role is to really discern … what is this about? I'm not there to dispute their experience, but to get at the root cause."
The four key symptoms of demonic possession have remained consistent for centuries. Along with a violent reaction to holy symbols or prayer, the afflicted person would show capabilities that defy explanation: superhuman strength, speaking or understanding a language previously unknown to the person, or knowledge of other information that he or she cannot possibly have.
Thomas, who pastors a church in Saratoga, Calif., said he has seen people rolling their eyes, hissing and spitting, and even assuming what he called a "reptilian look" in the cast of their facial features. His experience as a student and practitioner of exorcism is the subject of "The Rite," a book by the journalist Matt Baglio and the basis for a film scheduled to be released this month.
Thomas and others emphasize the extreme rarity of real cases of demonic possession. Of the 100 people he has counseled since he completed training in Rome five years ago, he said he performed a full exorcism for only five.
The rite itself - the subject of the third part of the Baltimore gathering - consists of a sequence of prayers, including the Lord's Prayer, the Litany of the Saints, other selections from scripture and prayers written specifically for exorcism.
The rite, which was revised in 1999 for the first time since 1614, takes from 45 minutes to an hour to complete. It is usually repeated, in a process that can unfold over months and even years, not unlike treatment with a therapist.