Do humans always accept proofs when presented to them?

[Mystic]

 Ok a non-Muslims studies Islam. Decides to become Muslim. Why would he read the Quran but not see the clarification of this issue if it was clarified? What bias does he have towards Sunnism when he doesn't come from that sect?

How can confirmation bias play a role here? 

Let's even go further. A Shia has confirmation bias to see his sect is right? Why would many read the Quran but not see the clarification of this issue in many of it's instances if it was clarified in those instances?

Either the issue is not clarified in those verses or this something else then bias going on.

A perfect example of your ignorance.  I tell you cognitive bias plays a role and because you only know about the operation of a few cognitive biases, and particularly confirmation bias, you exclaim that cognitive bias can't explain it, therefore dark forces is a better explanation.  There are hundreds of cognitive biases and tricks the mind plays on itself.  That you think 'confirmation bias' represents the whole field is simply ignorance.  And as pointed out in another thread, concluding 'magic' because you personally aren't smart enough to explain it is an argument from ignorance.  You're making judgements about cognitive bias and human psychology which you are not competent to be making.

You are right, I will have to look more into cognitive bias.