But no two people will respond to the same trauma identically, however. Another thinker of the period who has also achieved great influence in our own time was the “fighting scholar” Ahmed ibn Taymiyyah (1263–1382), also a refugee who, unlike Rumi, hated the Mongols. He saw the Mongol converts, now fellow Muslims, as kufar (“infidels”). He also disapproved of the suspension of ijtihad: in these fearful times jurists needed to think creatively and adapt Shariah to the fact that the ummah had been weakened by two ruthless enemies: the Crusaders and the Mongols. True, the Crusaders seemed a spent force, but the Mongols might still attempt the conquest of the Levant. In preparation for a military jihad to defend their lands, Ibn Taymiyyah urged Muslims to engage in the Greater Jihad and return to the pure Islam of the Prophet’s time, ridding themselves of such inauthentic practices as philosophy (falsafah), Sufi mysticism, Shiism, and the veneration of saints and their tombs. Muslims who persisted in these false devotions were no better than infidels. When Ghazan Khan, the first of the Mongol chieftains to convert to Islam, invaded Syria in 1299, Ibn Taymiyyah issued a fatwa (“legal ruling”) declaring that despite their conversion to Islam, the Mongols were infidels, because they observed the Yasa instead of the Shariah, and their Muslim subjects were not bound to obey them. Muslims had traditionally been wary of condemning fellow Muslims as apostates, because they believed that only God could read a person’s heart. The practice of takfir, declaring that a fellow Muslim has apostatized, would take on new life in our own times, when Muslims have once again felt threatened by foreign powers."
— Karen Armstrong, Fields of Blood - Religion and the History of Violence, Vintage 2015, pp. 200-201