Would you be suprised to know that there are 4 billion oral learers in the world? These are people whose primary way of communication is through oral formats, as opposed to literate. Want to learn more? Check out the International Orality Network. One man who has published quite a bit on this topic is Dr. David Sills. Some of his articles can be found here!
Just ask yourself: how would you teach the Bible and preach to a group of people who were not literate? It's a tough question if you are used to people taking notes while you teach!