There's a review of this book, HERE. Bart goes on to suggest that:
Given the nature of Christianity from the outset, as a religion that stressed proper belief and that required authorities on which to base that belief, literary texts very soon took on unusual importance for this religion. The apostles of Jesus were seen as authoritative sources of knowledge about what Jesus himself said and did. But apostles could not be present everywhere at once in the churches scattered throughout the empire. Apostolic writing therefore had to take the place of an apostolic presence and so the written word became a matter of real importance.
 Ehrman, Lost Christianities, pg. 231