Takfir is the process by which one Muslim characterizes another Muslim as a kafir. This designation carries with it a death sentence, according to certain interpretations of Islam.

The origins of takfir date back to the late 7th century, when the early Kharawij sect broke off from Sunni Muslims to commit wanton takfir on members of the Umayyad Caliphate and justify their indiscriminate attacks. In the 14th century, Salafi scholar Ibn Taymiyyah re-popularized the controversial practice of takfir in his effort to condemn the Mongol invaders en masse. Many Islamic scholars have treated the act of takfir with significantly more care, as the accusation—itself serious and controversial—is believed to require high evidentiary standards. A false accusation of takfir may invalidate the belief of the accusing party, according to some interpretations of Islam.

In the 18th century, Muhammad ibn Abd al-Wahhab—founder of the Wahhabi movement in Saudi Arabia—again normalized the controversial practice of takfir, encouraging a stringent Salafi interpretation of Islam and allowing for the possibility of excommunication toward Muslims that failed to abide by Wahhabi-Salafi practices. In the mid-20th century, extremist theologian Sayyid Qutb also encouraged takfir, this time as a means to justifying Islamist violence against fellow Muslims, including government leaders in the Middle East and North Africa. Islamic extremist groups like al-Qaeda and ISIS today rely on highly controversial interpretations of takfir in an attempt to justify assassination campaigns and terrorist attacks.