Brynne Larson and Tess and Savannah Scherkenback are all-American girls from Arizona, who enjoy martial arts and horse riding. But something sets them apart from most teenagers - they perform public exorcisms and often appear on TV chat shows.

Eighteen-year-old Brynne met sisters Tess and Savannah about eight years ago at a karate class.

"We just really hit it off," says Savannah, aged 21. "I don't know what happens but somehow you bond when you're punching one another and throwing knees. We were working out together, learning how to fight and how to defend ourselves and defend others."

They are now karate black belts, but because of their particular Christian beliefs, they have also decided to do battle - they say - against evil spirits or demons. They believe that these demons can possess a human being and cause suffering, depression or addiction.

"A demon can't just come into anybody whenever it chooses to - God doesn't allow that," says Brynne.

Media playback is unsupported on your device Media caption The teenage exorcists are skilled in martial arts and black belts in karate

"What happens is when someone sins or does something, or something's done to them that allows the demon to come into them, that's called the legal right or the reason that it's there."

The young women have been trained by Brynne's father, the Reverend Bob Larson, who says he has performed more than 15,000 exorcisms. They have appeared alongside him in America and overseas, including the UK.

What do the teenage exorcists believe? That exorcism can cast out a demon or evil spirit which has possessed a person

That these demons have names like Abaddon and Jezebel

That God cast the angel Lucifer out of heaven and he became Satan

That the angels who were cast out of heaven with Lucifer became demons

That a demon can be provoked to come to the surface, what they call "manifesting"

"Every single country has a specific kind of demon," says Tess, 18, who loves music and reading.

They believe that the UK in particular is a hotbed for "witchcraft", because of the popularity of J K Rowling's Harry Potter books.

"The spells and things that you're reading in the Harry Potter books, those aren't just something that are made up, those are actual spells. Those are things that came from witchcraft books," says Tess.

The teens see themselves as "freedom fighters". During an exorcism they brandish silver crosses and Bibles while confronting the so-called demon to make it return to hell.

"I'd have to say my special skills [are] probably the enforcer or the lead exorcist. I can take on those demons," says Brynne.

The teenage exorcists are greeted on stage as if they were celebrities. There is applause and they announce to the audience that they look forward to "kicking some demon butt".

But Brynne denies that it's a theatrical performance.

"Honestly, I've never tried to do any showmanship at all, I'm just demonstrating the power of God," says Brynne. "We're not doing it to play up to the cameras. I've seen some amazing things in private with nobody there."

The young women's mentor, Brynne's father, disagrees with critics who say it's dangerous to teach teenagers to perform exorcisms.

"We think it's OK to train teenagers to get drunk and have sex, but to do moral things for God, oh let's not train them to do that," says Larson.

He asks for a voluntary donation of a couple of hundred dollars or pounds when he and the girls perform a one-to-one exorcism, and rejects the idea that spiritual services have to be free of charge.

"Money and motives, anytime someone is in God's work it always comes up," he says.

"People will pay thousands of dollars to go to drug rehab or to pay psychiatrists but there's this idea that spirituality needs to be for free.

"The average megachurch pastor in America, it's not uncommon for them to make up to $1m a year. Well I can assure you we are nowhere near that."

Media playback is unsupported on your device Media caption Brynne, Tess and Savannah perform an exorcism in Ukraine

Exorcism is an ancient practice and one that appears in many different religions, but many believers doubt the existence of demons.

Sigmund Freud, the founder of psychoanalysis, saw spirit possession as a neurotic delusion, and demons as repressed "instinctual impulses".

Before undertaking a one-to-one exorcism, Larson says he asks clients to complete a psychiatric questionnaire to identify if they have any mental health problems. He says it is important for anyone with mental health problems to receive medical care and psychological support.

I don't really have time to go to school, but I'll just sit at my desk and work on calculus or read all my books Brynne Larson

Brynne's mother Laura Larson, says the girls are sincerely trying to help people.

"This is a family who lives by what they believe," says Laura, "and I think the teenage exorcists are making a difference, whether you believe in what they do or not, they are committed and they stand by what they believe."

She did not actively encourage her daughter Brynne to become an exorcist she says.

"Given the environment you would think that we would have been grooming them at the table, 'OK now how do you define this? What kind of demon is this?' But that really wasn't the approach.

"In fact I was the opposite, I didn't really keep her from doing deliverances but I didn't encourage her.

"Then her friends Tess and Savannah just started getting this training and she said, 'Mum can I please go along, I really want to learn,' and of course I mean I couldn't say no".

Tess, Savannah and Brynne have all been home-schooled. In Brynne's case it was because her father's profession led the family to travel widely.

"With going [to] over 20 countries and stuff, I don't really have time to go to school, but I'll just sit at my desk and work on calculus or read all my books," she says. "This is so much better than going to a stinky old school room."

Brynne and Tess have been awarded places at college this year, while Savannah is already a college student. But as well as continuing their education they are determined to continue their spiritual battle against what they see as the forces of evil.

Teen Exorcists will be broadcast on Thursday 12 September at 21:00 BST on BBC Three. Or catch up later on BBC iPlayer.

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