Medieval Christians first classified Islam as heresy within their own taxonomy

Evidence

As Christians began to learn about Islam and to take it seriously as a spiritual threat, they tried to come to grips with it in Christian theological terms. They studied it in order to place it in their preestablished taxonomy of error, the better to refute it. For the great majority of these authors, from John of Damascus in the eighth century to Paul Alvarus in the ninth, Islam was a heresy. — John V. Tolan, Saracens, 738

Categorization

• DEFINE: Explains the deep root of the ‘Islam as heresy’ framing: a preexisting taxonomy of error that new spiritual rivals had to be fitted into.
• XREF: connects to: Whether Islam technically counted as heresy was debated among medieval Latin theologians; Medieval theologians classified Islam in multiple overlapping religious frameworks; Orthodox Islam and Christianity hold mutual heresy accusations rooted in Crusade history — Aijaz Ahmad, In Theory: Classes, Nations, Literatures, 784