Illiteracy and Catholicism in the Middle Ages?

While I can't give you stats, I know the Norse nations had the highest rate of literacy (in Europe) in the dark and early middle ages due to the emphasis on runes in their faith. A century after the conversion of the Norse was complete, the literacy rate dropped to levels comparable to the rest of Europe. In Christendom, primarily only monks and priests were literate, but the Norse faith requires that everyone knew the Runic alphabet. So it seems to me that the "lapses" are more due to the Catholic church than religion in general.