Monsignor Quixote, Graham Greene
Another book I read (or in this case, re-read) while writing Escape From Fred. Once you've read it, you can see how this book might have informed it. Greene was my favorite author until I discovered Robertson Davies.
p. 55. It’s odd how sharing a sense of doubt can bring two men together perhaps even more that sharing a faith. The believer will fight another believer over a shade of difference; the doubter fights only with himself.