It is not the only, nor probably the best, reason for a person to leave their religion, but Dawkins summarized the very basic reason why I left religion (page 282):
Fundamentalists know they are right because they have read the truth in a holy book and they know, in advance, that nothing will budge them from their belief. The truth of the holy book is an axiom, not the end product of a process of reasoning. The book is true, and if the evidence seems to contradict it, it is the evidence that must be thrown out, not the book.Despite my honest belief, I came to realize that I valued reasoning over axioms. It is simply the more responsible approach. And when that reasoning does not lead to one's professed axioms, deconversion is virtually inevitable. These are my own words from early 2001 (note the similarity to those of Dawkins' above):
There is nothing wrong with believing a doctrine, the will to believe requires no justification, but if a doctrine is considered inviolable against the inquisitive nature, the very thing that suggested its possibility in the first place, then any further possibilities that are suggested will have to either accord with it or be rejected, although it may be more appropriate to question the primacy of the doctrine itself.Dawkins makes reference on several occasions to an idea that Dennett put forth: belief in belief. For some time after I wrote that, and even now, though I do not believe as I did before, I do at times believe in some form of belief. It isn't that I dislike belief, so much as that I earnestly value reasoning. (And as it is wont to do, reasoning often stands at odds with cherished beliefs.) Anyone one who feels the same will find a kindred spirit in this book.