The average christian (to use the most familiar example to me) doesn't believe that the bible is literally true all the way through. But why not? and how do they choose which bits to believe and which bits to call allegorical or just ignore?
A subject of perennial interest to atheists since it seems so arbitrary.
Here is my pet theory on the subject.
They don't.
Or at least there is no conscious and deliberate selection process.
Most beliefs are formed not from reading the relevant special book but from being brought up to believe, or running into some other strong emotional stimulus. So generally the belief is formed independently of the content of the special book, and, since times have changed considerably since most of the special books were written, the resulting belief is not likely to fit all the descriptions in the special book.
So, as the believer reads the special book, some passages fit their belief, and some do not.
Essentially a version of the logical fallacy of begging the question (in which there is no reason to believe one or more of the premises of an argument unless you already believe the conclusion).
And because fallacies are such beguiling things that they slip past conscious critical faculties the believer will not be engaged in a process of thinking "should I believe this passage?", rather it will just seem obvious to them which bits to believe and which not to. Also, since it is a process they are not aware of, when questioned they will only be able to say that it is obvious, or a matter or faith, or revelation, or something of that ilk.
