takfiir
Charging fellow Muslims with unbelief (takfiir) is an enormity in the eyes of Allaah. It is that fitnah (strife) which destroyed previous nations, and whose marauding fire in classical Islamic times was put out with the crushing defeat of the Kharijites, only to be revived on a wholesale scale more than a thousand years later by the Wahhabi sect of Arabia in the eighteenth century, from whence it has spread like a cancer today to a great many, otherwise normative Ahl-us-Sunnah, Muslim communities, becoming the pre-eminent bid‘ah (corruptive innovation) of our times, and one of the most destructive forces in the Muslim world.