Steve Ray has a list of 35 loaded Questions for "Bible Christians" (quotation marks his)(link to the whole list). This is number 25/35. I'm trying to provide the answers in a common format, for easy reference.
25) If Christianity is a “book religion,” how did it flourish during the first 1500 years of Church history when the vast majority of people were illiterate?
1) Literacy was actually much higher in the Roman empire in the early part of that period than Mr. Ray seems to think. Yes, toward the end of the medieval period in Europe literacy got low among the poor.
2) Those who could not read could still listen to the book being read.
3) Although the old churches had various liturgies, all of those liturgies read and sung Scripture.
4) It's hard really to view the Western church of the late medieval period as "flourishing" in anything other than numbers.