Medieval churchmen genuinely debated whether Islam was heresy

Evidence

Yet in spite of this widespread image of Islam as the culmination or summit of all heresy, it was not at all certain that technically it was a heresy at all. The clearest assertion that it was so was Oliver’s; because of their Christological beliefs, he said, Muslims ought to be called heretics instead of Saracens, but ‘the use of the wrong name has prevailed’. This flat statement is unique, but Peter the Venerable discussed the question fully. It was in the context of the catalogue of the heresies of the past that he came to the problem. ‘I cannot clearly decide’, he admitted, ‘whether the Muhammadan error should be said to be a heresy, and whether its sectaries should be called heretics or heathens.’ There were reasons to prefer the name of heretic. It might be correct to define as heretical nothing but what left the Church and acted against it, but ancient usage called any attack on some part of faith, even outside the Church, heresy, and Muslims, in the usual way of heretics, took what they liked from the sum of Christian doctrine, and rejected as much as they disliked. — Norman Daniel, Islam and the West, 1036

Categorization

• XREF: Connects to medieval Christian-Muslim polemics and how religious categories get contested at boundaries. — connects to: Whether Islam technically counted as heresy was debated among medieval Latin theologians; Medieval polemicists called Islam a heresy despite its unbaptised status.; Orthodox Islam and Christianity hold mutual heresy accusations rooted in Crusade history — Aijaz Ahmad, In Theory: Classes, Nations, Literatures, 784
• DEEPTHINKING: The technical question of whether Islam counts as heresy reveals how a rigid taxonomy (heretic vs heathen) strains under a religion that shares scripture with Christianity.