Medieval Europe understood Islam through its existing category of the Jews

Evidence

But it “could get no help from antiquity, and no comfort from the present. For an age avowedly dependent on the past for its materials, this was a serious matter. Intellectually the nearest parallel to the position oflslam was the position of the Jews. They shared many of the same tenets and brought forward many of the same objections to Christianity.” — Gil Anidjar, The Jew, the Arab, 213

Categorization

• XREF: This is the load-bearing causal claim: Islam was cognitively assimilated to Judaism because both challenged Christianity similarly.