Takfir is the act of declaring another Muslim to be outside Islam, and the classical jurists treated it as dangerous. The standard hadith warns that if a man calls his brother an unbeliever, the charge returns to one of them — so a wrong takfir falls on the accuser. Al-Ghazali wrote that the error of leaving a thousand unbelievers alive is lighter than the error of shedding the blood of a single Muslim. The whole apparatus was built to make expulsion nearly impossible. The Kharijites in the first century broke that rule and were the first to kill Muslims for insufficient belief, which is why the term is still used for the logic. Modern militant movements — from Egyptian jihadist groups through al-Qaeda to ISIS — revived and industrialised it, using takfir to license mass killing of Muslims, who make up the overwhelming majority of their victims. The 2005 Amman Message was the establishment's direct answer: convened by Jordan, endorsed by hundreds of scholars including the heads of Al-Azhar and Shia authorities, it recognised the eight schools of law as valid, forbade takfir against any of their adherents, and set out who is qualified to issue a fatwa at all. It has had limited effect on the people it was aimed at and considerable effect as a reference point.