The Gutenberg bible was important because it was one of the first printed books. The printing press later proved immensely useful in spreading knowledge and faith. Just 70 years later the Luther Bible went on to be sold by millions and become a part of virtually every German-speaking Protestant’s home.

And you really don't need for people to be literate to convert to a religion. For most of human history the majority of people were illiterate. It was spread by word of mouth and churches were the place of worship.

Also I think it is important to note that throughout this period a significant proportion of the population, although a small number at first, were literate.

Furthermore this number rose throughout the century as printed book became more prominant.