I’m reading a lot of Middle English romances right now, and there are multiple references to Muslims (Saracens) worshipping idols and multiple pagan gods.
For example, King of Tars (c. 1330 CE), ll. 496-500 “Sche kist Mahoun [Mohammed] and Apolin [Apollo], / Astirot [Astarthe?] and Sir Jovin [Jove] /.../ ...sche was in the temple / Of Ternagent [similar to Allah] and Jubiter [Jupiter].”
Was this belief widespread and was it genuinely held by the educated, or is it a type of anti-Muslim propaganda?
A lot more can be said about this topic - I don't know about the Middle English romances specifically, so maybe someone can answer that part better, but I've answered a few other questions about medieval European beliefs about Islam:
I'm a Crusader heading towards the Holy Land in 1096. How much do I understand about Islam?
Did Muslims worship a silver idol of Muhammad in Jerusalem
There is also another answer by u/sunagainstgold to a similar question:
Hopefully those help!