[…] The religion allows many interpretations, and Islamic State supporters are morally on the hook for the one they choose. And yet simply denouncing the Islamic State as un-Islamic can be counterproductive, especially if those who hear the message have read the holy texts and seen the endorsement of many of the caliphate’s practices written plainly within them.
Muslims can say that slavery is not legitimate now, and that crucifixion is wrong at this historical juncture. Many say precisely this. But they cannot condemn slavery or crucifixion outright without contradicting the Koran and the example of the Prophet. “The only principled ground that the Islamic State’s opponents could take is to say that certain core texts and traditional teachings of Islam are no longer valid,” Bernard Haykel says. That really would be an act of apostasy.
[…] Non-muslims cannot tell Muslims how to practice their religion properly. But Muslims have long since begun this debate within their own ranks. “You have to have standards,” Anjem Choudary told me. “Somebody could claim to be a Muslim, but if he believes in homosexuality or drinking alcohol, then he is not a Muslim. There is no such thing as a nonpracticing vegetarian.”
The Atlantic, Graeme Wood: “What ISIS Really Wants”.