Beating the truth out of her.

Here is a scary trip through religion, faith, and superstition. Be ready to be creeped out.

Last year, I met a drawn, defeated 14-year old girl who had been possessed by Satan, until he and his Armies of Evil were tortured out of her.

That is how her priest explained it to me. That is how she explained it to me.

They spoke as if it was all as obvious as her scars. Clarice was a tiny girl wrapped in a big white woollen cardigan. In a church in the middle of Congo's carnage she explained how she had chosen to let the demons enter her when she was twelve.

Since then, Satan had forced her to make her mother fall, breaking her leg.

Satan had forced her to jinx her father, making it impossible for him to get a job. Satan had forced her to kill her little sister, by giving her a deadly fever.

Her Pentecostalist priest, Papa Enoch Boonga, told me with pride how he had driven the demons out. They starved Clarice for four days, whipped her and threatened to burn her, until finally she "confessed."

Then they forced her to admit to everything she had done, and performed a long exorcism ceremony. They only believed it was working when her little body began to judder and howl and curse. I ask Clarice quietly if she really believed she had done all these things. "Yes," she said. "I do." And so we sigh lazily: another example of African primitivism. But no. Exorcism, even of children, is being aggressively promoted today by one of the most powerful men in the Western world, Pope Benedict XVI, in only slightly watered-down form. Presidents and Prime Ministers fawn over this man.

His every word is reported with the respect we are required to show to "religion", lest we are accused of bigotry.

And yet he is openly commanding an army of exorcists to tell horrifically disturbed and mentally ill people that demons and devils are within them – because they invited evil in.

This December, Father Gabriele Amorth, official exorcist of the Rome Diocese, and friend of the "Holy Father", announced that the Pope will soon undertake a new campaign to unleash a fresh batch of 400 exorcists on the world, in addition to the thousands already in operation. "Thank God," he said, "we have a Pope who has decided to confront the devil head-on." (One representative of this evil, he added, is Harry Potter.)

The Catholic Church has officially denied having such a plan – but they admit the Church has had official exorcism rites since 1614, and that bishops and priests are encouraged to act on them today "where appropriate."

In practice, official Catholic exorcisms have been dramatically increasing since the mid-1970s, according to Michael W. Cuneo, a sociologist at Fordham University in New York who conducted a four-year study into this topic. But very few people were prepared to talk to me about what this involves.

I was finally able to track down one of the more "moderate" exorcists – if you can imagine such a thing – in the Roman Catholic Diocese of Hallam, in Yorkshire.

Father Anthony Hayne is a soft-spoken, sincere man who tells me that in his little slice of Britain he carries out "a couple of exorcisms a month".

Although it's years since he's seen the movie The Exorcist, he thinks it's broadly accurate, explaining, "As a Catholic, I would use those methods, yes.

"Lately I've been seeing a professional person who seems very balanced, except she feels demons are ruining her life. Sometimes she speaks with a voice that is certainly not her voice, it is obviously a demon using her. . . We have prayed together, and if that doesn't work we have a solemn rite of exorcism we can use if we have the approval of the bishop."

He says he takes the question of whether these people are mentally ill "very seriously".

He encourages anybody who feels they are possessed to go to their doctor – but he claims he has to take their word for it when they say the doctor says there is no physical or psychological cause for their disturbance, because of patient confidentiality.

Father Anthony tells me he does indeed believe children can be possessed, although he would only ever "treat" a child with the presence and co-operation of its parents. The closest he has come is to treat a few people in their late teens, who "had been using ouija boards and had let the darkness into their lives".

It's true the official Catholic exorcism rites do not involve the physical torture inflicted on Clarice – but in many cases, it inevitably involves telling the mentally ill they are responsible for their own suffering.

For example, one of the heroes to Catholic exponents of exorcism is M. Scott Peck, who wrote: "People who are suffering from demonic possession [have] at some level co-operated with demonic evil; they have invited it into their life. In such cases there is always – perhaps at an unconscious level – some kind of sell-out to evil." Imagine telling this to a howling woman hearing voices. Every day there is a case somewhere of an exorcist taking the Catholic theology seriously but going beyond its fetid rules. To pluck one recent example from hundreds: recently a 23-year-old Romanian nun called Maricica Irina Cornici became convinced she was hearing the voice of Satan.

Her colleagues reacted by tying her to a cross, gagging her mouth with a towel, and leaving her for three days with no food or water, to "starve out Satan". At the inquest, it was revealed she had a long history of schizophrenia. Whenever I write about the harm caused by organised superstition, well-meaning people write, asking – why get so worked up? Isn't religion all about love and light and helping people through the hard times?

No. Here is the biggest, richest and most powerful religious institution on earth, explicitly telling deeply mentally ill people the Devil is within them because they asked him in, and they have to suffer to get him out. Even the famously wet and woolly Church of England has an official exorcist in every diocese. This is not love, nor light.

Nor can we fall back on the glib claim that these people are distorting the "true" religion, which is all about peace and love. Jesus Christ himself performed exorcisms and he called on his followers to copy him, engaging in spiritual trench warfare against Satan. If a Christian is somebody who follows the example of Christ, these people are true and good Christians – and they are also deeply immoral human beings.

In that crater-church in Congo, I wanted to be able to hug Clarice, and tell her that there was a place in the world where these barbaric superstitions were a distant, dismissed relic. Thanks to the Catholic Church, I could not offer her even that.