Kharijites

The Khawarij (Arabic: الخوارج‎‎, al-Khawārij, singular خارجي, khārijiyy) or the ash-Shurah (Arabic: الشراة‎, translit. ash-Shurāh‎ "the Exchangers") are members of a group that appeared in the first century of Islam during the First Fitna, the crisis of leadership after the death of Muhammad. It broke into revolt against the authority of the Rashidun Caliph Ali after he agreed to arbitration with his rival, Muawiyah I, to decide the succession to the Caliphate following the Battle of Siffin (657). A Khariji later assassinated Ali, and for hundreds of years, the Khawarij were a source of insurrection against the Caliphate.

The Khawarij opposed arbitration as a means to choose a new ruler on the grounds that "judgement belongs to God alone". They considered arbitration a means for people to make decisions while the victor in a battle was determined by God. They believed that any Muslim—even if not Quraysh or even an Arab—could be the Imam, the leader of the community, if he was morally irreproachable. If the leader sinned, it was the duty of Muslims to oppose and depose him.

Some Khawarij developed extreme doctrines that set them apart from both mainstream Sunni and Shiʿi Muslims. They were particularly noted for adopting a radical approach to takfir (declaring self-described Muslims as non-Muslims). In modern times, the Islamic State of Iraq and the Levant and some other extremist groups have been referred to as "Kharijites" for their willingness to commit takfir and kill those they deem insufficiently Muslim.

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Wikipedia