In The Song of Roland, a few different French characters make reference to the Muslim Saracens worshipping Apollo. Is this something Europeans at the time believed happened? If so, why? And if not, why was it included in this story?
11th century Christians in Europe who had very little or no contact with Muslims, and sometimes even the ones in parts of Europe where they had lots of contact with Muslims, pretty much just didn't care at all what Muslims actually were or what they believed. They couldn't fit Muslims into the pagans-Jews-Christians paradigm. They couldn't be a new religion, because how could there be any new prophets after Jesus? They obviously weren't Jews, so maybe they were simply Christian heretics? Or maybe they were a previously-unknown kind of pagan? And if they were pagan they probably worshipped pagan gods like Apollo.
In addition to the previous answers from sunagainstgold, here's something I wrote about a similar question:
I'm a Crusader heading towards the Holy Land in 1096. How much do I understand about Islam? (part 1) (part 2)
And on the other hand, here is what one 12th-century crusader knew about Islam