I would guess that someone has argued of the contents of a book that did not exist at some point. There has probably been an author who died, and there were rumors that an unpublished book existed; fans of the author may then have argued over what the book would be about, or what would occur in it, even though that book may not have actually been more than rumor.
I'm not sure how this relates to god. The concept of a god clearly exists, so some argue for it or against it; were you trying to say that if the idea of god didn't exist, no one would be arguing about it?