Title basically says it all. Since Catholic Mass was in Latin and (I believe) most people wouldn't have been literate, how much did your ordinary person in Medieval Europe understand about the theological details of Christianity? Did they have an understanding of things like the link between Jesus' sacrifice and the Fall in the Garden of Eden?

I recall a (Catholic) English teacher in High School saying that for a long time Catholicism was verging on polytheism, as Saints with different domains of influence took a larger role in day-to-day spirituality, but I have no idea if that's true.