Medieval Christians framed Islam as an old heresy rather than a new religion

Evidence

It was natural for Christian thinkers to interpret the unfamiliar and unexpected phenomenon of Islam in terms of what was familiar to them already. Therefore they tended to present Islam to themselves not as a new religion, but rather as the variant of an old heresy. … As Richard Southern has observed: ‘There was only one way in which this chaos (as it must have appeared) of truth and falsehood could be treated by a twelfth-century thinker if it were to be brought within the range of his equipment: it must be treated as a heresy, more mysterious in its operation than other heresies which had appeared in the course of Christian history, but like them a more or less deliberate perversion of the true faith, which required refutation by the ordinary rules of argument.’ — Robert Irwin, For Lust of Knowing, 48

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. It thus should not come as a surprise that when twelfthcentury Latin authors tried to come to terms with Islam, they also classified it as heresy and tried to refute it using the well-worn tools of antiheretical argument. — John V. Tolan, Saracens, 738

Categorization

• DEFINE: Explains the ‘Islam as heresy’ interpretive framework that dominated Christian thinking for a millennium.
• XREF: Directly genealogically grounds the vault claim that anti-Islam discourse developed out of anti-heresy/anti-Judaism frameworks rather than from genuine encounter with Islam. — connects to: Medieval Christians imagined Islam as a Christian apostasy born from lost provinces; Medieval Christians saw Islam as salvation by faith without works; Whether Islam technically counted as heresy was debated among medieval Latin theologians (+3 more)
• ADDENDUM: Supports the genealogical method used across the vaulted Anidjar/Aziz pieces by showing the pre-Christian heresiology template into which Islam was slotted.
• corroborated: 2 independent passages support this claim — Robert Irwin, For Lust of Knowing: The Orientalists…, 48; John V. Tolan, Saracens, 738