Medieval Christians saw Islam as doubly heretical, overlapping both their faith and Judaism

Evidence

For the Christian, Islam was the third of three religions and heretical in its relation to both the other two. Christianity has its special relation to the synagogue, so that Islam was doubly related to Christianity, directly, and, through the old Law, indirectly. This was illustrated by an argument of Oliver’s, designed to show the relative status of Islam: in any assembly of a Christian, a Jew and a Muslim, the Christian and the Jew would alike regard Islam as the least of the three, the Muslim would put Christianity above Judaism. — Norman Daniel, Islam and the West, 1059

Categorization

• QUESTION: Oliver’s assembly argument that Muslims would rank Christianity above Judaism is surprising — does any Muslim source actually attest this ranking, or is it purely Christian projection?
• XREF: connects to: Medieval theologians classified Islam in multiple overlapping religious frameworks; Medieval authors mislabelled Islamic Christian and Jewish influences as heretical sects; Medieval polemicists placed Islam midway between Jewish and pagan error (+1 more)