Question

[Whateverist]

Khemikal.
You reckon that's why?  It's that they don't know how to defend some x?  I'd suspect that people leave their religions because they no longer believe in some x...even if they were aware of all the right verses to reference, personally.  Elephant in the room...though, are we sure that's when it starts?  

Good question.   I have considered this.  A person can have vast amounts of "head knowledge"  .  Like you said they may even know the right verses of whatever to reference to actually defend.  If they walk away so easily or not so easily, did they really believe what they "believed" in the first place?  Does that even make sense?  
Eeh,  got to get off here but just put this subject out here as far as college level.  A psychology professor told her class couldnt talk about a god.

*my bolding*

I wouldn't be too surprised to learn that an individual professor might say or do something that seems ill considered or heavy handed.  Every group is made up of individuals with their own strengths and weaknesses.

Of course appeals to authority as justification for an argument are discouraged and down graded in most any college course involving ideas.  This is an example of how the excessive and irrational commitment of Christians to the bible runs counter to what higher education is all about.  Quoting scripture to argue for a secular point doesn't actually support that point.  All it does is make the case that all people who fetishize the bible should agree with you, and that doesn't matter in a secular setting.