The purpose of When Prophecy Fails is to explain the psychological idea of cognitive dissonance by using a small cult as an example. But in fact I found this book to be one of the insightful explanations of religion that I have read. It is particularly relevant to Christianity.
One obvious lesson of this book is that hysterical women are masters of rationalization, and that such rationalizations can form the basis of religion. This clearly applies to the cult described in this book, and also to Christianity.
Another key lesson of this book, recognized by the author, is that personal contact is essential to religion. No religious group can be formed on the internet because of the lack of personal contact.