Takfir's history is bracketed by two insurgencies thirteen centuries apart. The Kharijites, the faction that seceded from Ali's camp after the arbitration at Siffin in 657, built their movement on an equation: the grave sinner is an unbeliever, and the unbelieving ruler and his followers may be fought and killed. Their assassins struck down Ali himself in 661. Mainstream theology defined itself against exactly this: the Murjia deferred judgment on the sinner to God, and the consolidating Sunni consensus held that sin does not expel from the faith, making anti-Kharijism part of orthodoxy's foundation.
The equation returned in the twentieth century. Sayyid Qutb's prison writings described contemporary Muslim societies as jahiliyya, pre-Islamic ignorance; the takfiri groups that followed, from the Egyptian Takfir wal-Hijra of the 1970s through the movements behind Sadat's assassination in 1981, to al-Qaeda and ISIS, extended the charge from rulers to armies, police, sects and eventually whole populations, licensing the violence the charge entails.
The response of institutional Islam has been unusually univocal: the Amman Message of 2004 to 2005, endorsed by hundreds of scholars across all eight recognized schools of law, forbade takfir of any adherent of those schools, and the 2014 open letter of over a hundred senior scholars to ISIS made the classical restraints its centerpiece. The debate over takfir is thus not a debate between Islam and its critics; it is the tradition's oldest internal argument, resumed under modern conditions, about who, if anyone, holds the key to the boundary.