Both medieval Christianity and Islam defined each other by disagreement, not overlap

Evidence

The theoretic assessment remains clear. Islam was a falling away from the truth which the Christian Church preserves; and in this way was paralleled the Islamic concept of Christianity as a falling short from the truth preserved in Islam. On both sides it was the extent of disagreement, not of shared beliefs, on which men concentrated their attention. — Norman Daniel, Islam and the West, 1080

Categorization

• XREF: Mirrors-modern phenomena: partisan identity defined by opposition; connects to the polarization findings in the reader’s knowledge base. — connects to: Medieval Christians understood Islam only as a distorted copy of Christianity; Christian-Muslim controversy necessarily centers on their fixed points of doctrinal disagreement; Orthodox Islam and Christianity each deemed the other’s core doctrine heretical — Aijaz Ahmad, In Theory: Classes, Nations, Literatures, 784