Medieval Christian suspicions about Muslim crypto-polytheism drew on Jewish sources.

Evidence

There does seem to have been a genuine lingering suspicion in some quarters that Muslims were only incompletely de-paganised, despite their ostensible teaching, and even that they actually were crypto-polytheists. This point was made some ten years ago by Grégoire in his stimulating discussion of the problem. He concentrated on the idea that the cults of the Jahiliyah were deliberately preserved in the Ka’bah. He suggested that this suspicion might have entered Christendom from Rabbinic writers known to Pedro de Alfonso. — Norman Daniel, Islam and the West, 1766