Medieval Christians understood Islam as heresy and lumped Muslims with Jews and heretics.

Evidence

This caricature of Saracen paganism was untenable for those with even a rudimentary familiarity with Islam, many of whom portrayed Islam as a Christian heresy. For Guibert de Nogent, chronicling the first Crusade, Muhammad was merely the latest and most nefarious of a long line of oriental heresiarchs: the success of Islam was proof of the oriental penchant for heresy, calling for the intervention of vigorous and stolid Latins. The image of Islam as heresy, an image forged by dhimmis in the Near East and Spain, came to northern Europe at a time when Latin Christians were increasingly in contact with Muslims and when they were increasingly preoccupied with the supposedly nefarious influence of other non-Christians, Jews and heretics. The association between these various enemies of the faith is crucial for understanding the Christian perceptions of Muslims (or, for that matter, of Jews or heretics) in the following centuries. Petrus Alfonsi included an anti-Muslim chapter in his Dialogues against the Jews; Peter of Cluny composed a polemical triptych against Jews, Muslims, and Petrobrusian heretics; Alain de Lille, a treatise against Cathars, Waldensians, Jews and Muslims. — John V. Tolan, Saracens, 1360

Categorization

• XREF: Connects to the theme of Christians framing out-groups through a template of religious error, resonating with Tolan’s broader argument and with any history of heresy construction.