In the modern world, belief in Satan or evil spirits is usually ridiculed or completely ignored. Scientists often assume such ideas are delusions – symptoms of a psychiatric illness or psychosis. Some Christians even dismiss the idea, even though they feature prominently in the gospel accounts of Jesus's life.

So when a distinguished expert in mental illness, Dr Richard Gallagher, wrote a book describing his own personal encounters with people who are possessed, it received a lot of media attention, with TV appearances, newspaper interviews and appearances on many podcasts.

Demonic Foes: my 25 years as a psychiatrist investigating possessions, diabolic attacks, and the paranormal was published by Harper Collins in 2020 and the doctor has been in the media ever since. In the book, Dr Gallagher writes of "unmistakable cases of demonic possession and other diabolical attacks that I directly encountered over the past twenty five years."

He describes his work as a mainstream psychiatrist who was at first reluctant to get involved with demonic possession, but came to be regularly consulted by the International Association of Exorcists and other trained clergy. His role was to rule out standard psychiatric problems and offer advice to the priests involved, sometimes attending the exorcisms themselves.

His sober and objective descriptions of these eery events – including levitations, paranormal knowledge and speaking in unknown languages – make his account all the more convincing.

He details a number of these cases in the book. A particularly disturbing incident took place with 'Julia' (all names were changed and permission to publish sought by the publishers) who went to a Catholic priest for help after experiencing blackouts where she would speak in other languages, and she believed that she was possessed by evil spirits. However, she was a member of a satanic cult which she did not want to leave – so she did not receive relief despite repeated exorcism rituals.

During consultations with Dr Gallagher, Julia revealed knowledge that he is certain she had no way of knowing, such as what a priest was wearing hundreds of miles away – which was confirmed straight away by phone. These kind of uncanny incidents convinced him that it was a supernatural problem and not a psychiatric condition. "I always believe that skepticism should start governing all these investigations," he writes in the book. "But it is difficult and foolish to hold on to disbelief in the face of such overwhelming evidence."

In contrast to 'Julia', most of the cases he mentions are people who repented of occult activity or dabbling with serious evil in their past.

For example, he worked with a former gang member he called 'Juan' who had turned to "Santa Muerte", a hispanic occult practice, to seek the devil's help in his criminal and violent activities. However when finally imprisoned for drug offences, Juan found that he could not escape these dark spiritual activities, and he found himself entering into frightening trances and displaying superhuman strength when a prison chaplain attempted to perform a deliverance.

Back on the outside and married, Juan tried to turn to God, but he found himself being prevented against his will from entering churches. Dr Gallagher assisted in more exorcisms for Juan, and he started to find relief from his symptoms as he worked on growing his spiritual life.

As in this case, for freedom from possession, Dr Gallagher prescribes a genuine spiritual path. "You have to work at it," he told the Daily Mail. "You have to renounce your evil ways or your involvement with something nefarious like the occult, and you have to want to reform your life... you have to want to turn to God basically."

Dr Gallagher is no quack. He is still listed as a professor at New York Medical College, and lecturer at Columbia University. He was trained at Yale. His book features a foreword from a former president of the American Psychiatric Association, Dr Joseph T English, who describes his colleague as "superbly credentialed" and commends the book as "serious scholarly study". It reads: "Dr Gallagher brings his trained intellect and unimpeachable integrity to bear on the investigation of such 'states of possession'".

Dr Gallagher says that many other medics support his claims privately. "Readers may be surprised to learn that many physician colleagues of mine - around the world – agree with my findings, though they may be reluctant to speak out so openly," his book's introductory chapter reads.

As he says, it is not unusual for people who have psychiatric illnesses to describe strange spiritual phenomena. He does not consider this to be supernatural in origin, but simply the effects of a medical condition.

Current scientific explanations for such hallucinations and phenomena vary, but are biological rather than spiritual. Dr Gallagher is critical of those who attribute demonic activity to every human problem and mental illness, and warns against working with untrained people who claim to practise exorcism or deliverance.

In fact, despite his Catholic upbringing, at first he was sceptical of the reality of possession. However when a priest first came to him for help, he gradually observed enough evidence to convince him. But still he concludes possession only after he has ruled out standard organic, psychiatric causes.

He admitted to Esquire magazine that he was reluctant to 'go public' with these conclusions. "I wasn't sure that it was a great idea, professionally speaking, to publicise a belief that is unacceptable to a lot of people in my field," he told the publication. He persevered, he said, because he felt he could "enlighten" people.

Some of his insights are reassuring. For example, he believes genuine possession is rare. "The people who become possessed are almost invariably—and I'm choosing my words carefully here, because there are a few exceptional cases—people who have turned in a serious way to something evil," he told Esquire. This includes explicit worship of Satan or extreme violence and criminal activity.

"I especially make sure the patient isn't suffering from an odd seizure disorder or other undiscovered brain damage," he told the Daily Mail. "I rely upon a full narrative assessment and symptom survey that I always conduct. I also generally interview friends and families to confirm all details reported by the patient.

"Make no mistake, these attacks should not be haphazardly diagnosed; they require no less discernment as those made in medical practices."