First person: 'I have witnessed 20 exorcisms'

In Rome, exorcists have waiting rooms. It was one of many surprising things I found when I decided to write about an act whose popular image, seen in films, is of Gothic locations in some shadowy underworld. I'd read books written by fundamentalists who saw demons everywhere and others who only ridiculed the act. I wanted the truth. I read about a course being offered at a university affiliated with the Vatican and decided to find out more.

The course used PowerPoint presentations and the teachers had doctorates. I met an American priest called Father Gary Thomas, who had been sent to Rome at the behest of his bishop. I got to know him and ended up watching about 20 exorcisms he did. They were bizarre on many levels.

The theory goes that demons possess people, affecting them – as one person described it – as if from the end of a pipe. The exorcist's prayer stimulates the demons, forcing them through the pipe, as it were, and they take over the body. The demons try to get the exorcist to stop saying the prayer, often by causing dramatic behaviour. For example, because the demons can't look at the priest, the person's eyes roll up and become white or they close tight.

One case involved a nun. I wasn't in the room but I heard the voices through the door. It sounded like a dog trying to talk. Her voice had a quality to it that was definitely not human. She cursed the exorcist, laughing at him and telling him he had no power. Father Gary said this woman became unrecognisable. She flopped around on the floor like a fish and started smacking her head into a wall. For him, the suffering proved the behaviour was not faked.

Father Gary also saw a small woman of about 26, whose convulsions were so strong she bent the legs of a metal chair and on another occasion broke a wooden chair. In another case, I watched a woman who started belching really loudly, like a big guy in the pub. She started to vomit and spat something up. Another woman levitated, but probably the most powerful experience for Father Gary was a woman who was being held down and looked at him with open eyes, which is rare. He said he felt an evil presence trying to stare deep inside him.

The ritual lasts half an hour. When it ends, the demon detaches itself and goes back to being a spirit. It's a repeat process – the ritual diminishes the ability of the power of the demon until it can be said to have been cast out completely. Exorcism is not always violent. There are famous cases, perhaps most notoriously that of Anneliese Michel, a German who died as a result of exorcism, but another thing that surprised me was that the ritual is often very mild. And those who react strongly often come with other people. I saw one woman who was very violent but came with her parents and husband, who could restrain her. Usually, though, it's enough for the exorcist to place his hand on top of the person's head and say the prayer. There's no shouting or screaming – just a sense of calm.

Father Gary is currently seeing a scientist in order to exorcise him. The scientist doesn't believe in possession – so he's angry about what's happening to him. Many people assume exorcism involves only the very devout being brainwashed by priests. That goes on but I'm certain other cases go beyond our understanding.

Whether or not demons exist, I've talked to enough non-believing sceptics and doctors who admit something real is happening. That isn't to say I would recommend it.

'The Rite: The Making of a Modern Exorcist' by Matt Baglio (Simon & Schuster, £12.99)