Early modern polemicists classified Islam alongside heresies for systematic refutation

Evidence

The greatest of all was Grotius, whose general plan is very like that of Vives, although his style is incomparably more academic; he depended heavily upon the Cluniac corpus and quoted the Qur’an from Ketton. … The most thorough was the Dutch Reformed Hoornbeeck, who (1653) itemised separately and point by point the heresies of every sect, including Muslims, Papists, Lutherans, Brownists, Jews, Greeks and Pagans. — Norman Daniel, Islam and the West, 2443