Lalish, Iraq (CNN) Sitting on a shabby green sofa somewhere in the ISIS-controlled city of Mosul, Iraq, the militants laugh and joke as one of them films their excited chatter.

"Today is the female sex slave market day, which has been ordained," explains a skinny, black-clad Jihadi, gesturing at the camera.

"With Allah's permission, each will get a share," promises another of the fighters.

"Where is my Yazidi girl?" asks the first, a wide grin splitting his straggly-bearded face.

The Yazidis are an ancient people, followers of a unique religion that blends elements of Islam, Judaism and Christianity with even more ancient practices, including sun worship.

They believe in a single god who created the Earth and left it in the care of a peacock angel, Malak Ta'us.

Photos: Lalish targeted by ISIS Photos: Lalish targeted by ISIS Lalish, in northern Iraq's Nineveh Province, is the spiritual heartland for Yazidis. Hide Caption 1 of 6 Photos: Lalish targeted by ISIS Yazidis believe in one God as the creator of the world, but many have been raped or murdered by ISIS, which claims they are "devil worshipers." Hide Caption 2 of 6 Photos: Lalish targeted by ISIS The ancient Yazidi faith pre-dates Islam and Christianity. Hide Caption 3 of 6 Photos: Lalish targeted by ISIS About half a million people belong to the Yazidi community. Hide Caption 4 of 6 Photos: Lalish targeted by ISIS Thousands of Yazidis are still believed to be held by ISIS, in the parts of Syria and Iraq under its control. Hide Caption 5 of 6 Photos: Lalish targeted by ISIS Baba Sheikh, the spiritual leader of the Yazidis, says he hopes they will return home soon. "'I wish they all will come back -- we are going to welcome them warmly," he said. Hide Caption 6 of 6

But this belief -- decried as "devil worship" by ISIS -- has been used by the Islamic extremists to justify murder, enslavement and rape.

"They took our girls, our homes and our families," says Yazidi spiritual leader Baba Sheikh. "They took all of them. We say our fruitful generation is our children, but they took them all, young and old."

Noor (not her real name) was sold into slavery after ISIS overran her village in the Iraqi province of Sinjar. The 22-year-old says the militant who picked her out raped her -- but not before trying to justify himself.

"He showed me a letter and said, 'This shows any captured women will become Muslim if 10 ISIS fighters rape her.' There was a flag of ISIS and a picture of Abu Bakr Al Baghdadi."

After abusing her, he passed her on to 11 of his friends, who also raped her.

In ISIS territory, Yazidi women can be bought and sold for money, bartered for weapons, even given as a gift; but this is not a simple commercial transaction -- ISIS has made rape and slavery part and parcel of its -- brutal -- theology.

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"ISIS fighters told us, 'This is the rule of Abu Bakr al-Baghdadi and we must do it,'" Noor explains. "[They said] 'Anyone who doesn't convert to Islam, we will kill the males and marry the girls. They are the spoils of war. '"

In its online English magazine, Dabiq, ISIS lays out its justification for its brutality against the Yazidis on religious grounds:

"Enslaving the families of the kuffar [unbelievers] and taking their women as concubines is a firmly established aspect of the Shariah [Islamic law] that if anyone were to deny or mock, he would be denying or mocking the verses of the Qur'an and the narrations of the Prophet."

But theologians the world over point out that ISIS's actions have no basis in Islam.

"The people of ISIS don't represent Islam at all. In fact, if anything, they are anti-Islam," says London-based Imam Ajmal Masroor. "They have hijacked Islam. They have denigrated Islam. They have desecrated it."

"In Islam taking anyone as captive, mistreating them using them as sex slaves, torturing them and killing them is totally prohibited.

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"That's what God says in the Quran: 'Those people who lose their capacity to use their brain, their perceptive capacity to see and hear the truth, they are worse than animals.'

"That's exactly what they have demonstrated. There is no room for any discussion on this. It's haram [forbidden], it's anti-Islam and it should be treated as such."

For Yazidis, the tragedy is so great that their own strict traditions have had to adjust.

Before ISIS attacked Sinjar, marrying outside the Yazidi faith was strictly condemned. Those accused of adultery -- and even victims of rape -- could be killed for "dishonoring" their family.

But that is changing, according to Baba Sheikh, who desperately wants those abducted by ISIS to return.

"Anyone who comes back will be warmly welcomed home," he insists. "They should keep their heads up. They have done nothing wrong. And they should not be worried."

His words are a source of comfort for the tormented; even as ISIS attempts to destroy the Yazidi people in the name of religion, the terror group's victims may still find solace at home.