Few things demystify a faith or its sacred texts like learning about the haphazard historical processes by which traditions formed their beliefs, wrote their texts, and came to settle on them as authoritative. So many pieces of one's faith which the average parishioner takes as basic and unproblematic suddenly become problematized for the first time.
Worse yet, the more you see how your faith emerged out of a contingent historical process is the more that you can see the undeniably human fingerprints all over it. And these sorts of realizations start to undermine the sense of the perfect internal rationality of one's beliefs.
And when you are already dubious of absolute truth, the suspicion that your beliefs are not really absolutely true but a matter of the wrong presupposition, one you adopted more as an accident of where you were born or perhaps a profound experience you had, than because of any superior access to truth, the more you will begin to doubt.