The Khawarij were an early sect who rebelled against ʿAli ibn Abi Talib and pronounced Muslims disbelievers over major sins, then made their blood lawful. The Prophet ﷺ described them before they appeared: a people who recite the Qurʾan but it does not pass beyond their throats, who "pass out of Islam as an arrow passes through its game," outwardly devout yet astray. Ahl al-Sunnah oppose their two core errors — declaring sinners to be disbelievers and rebelling against the Muslim authority — and see their spirit revived in modern extremist takfiri movements. The refutation is not achieved by matching their zeal but by returning to correct knowledge and the understanding of the Companions.
Evidence — the daleel
Sahih Muslim 1064 ṢaḥīḥSahih Muslim 1066 Ṣaḥīḥ
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Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.