Reasons [rationality] (not common knowledge)?
Perhaps its not rational to believe in that which has never been shown to exist. Further, it may not be rational to lead ones life based on those beliefs. It may further be shown that it is irrational for some to control others based on those beliefs, although those in that power role may consider it completely rational, based on their own desires.
intellectual responsibility compells one to find out before arriving at conclusions
Agreed, but that doesn't appear to be the case.
You may refer to the overused flat earth and geocentric solar system analogies.
Proclaimed and protected ideals from a theistic thinking and decision making process. Please note the use of the scientific method coupled with critical thinking have dispelled those ideals.
Which human can claim a masterful understanding of reality, which may be clearly communicated to all?
A 'masterful' understanding of reality does not exist, but perhaps one day may, if science is allowed to pursue that understanding and not be hindered by religious ignorance.
God, I presume, should have a masterful understanding of reality, especially since he created it, and should therefore be quite able to convey that understanding to all.
How about the discrepancies regarding the nature of God?
"In the Bible God "draws near", "comes down" and seeks after us in order to enter into an intimate relationship with us. The whole Bible is the story of God seeking man.
In contrast, the Qur'an portrays God as one who is "far", who is transcendent only. As a Muslim theologian has said, "God reveals only his will, not himself. He remains forever hidden". Islam is about man trying to please God by obeying his will.
The Bible reveals God as taking the initiative and coming down to seek after us. The movement is in the opposite direction."
http://answering-islam.org.uk/lovesus.html
Maybe its there in front of our faces but we are just not looking at it (perceiving it?) in the correct way.
What exactly are we to perceive, and how are we to assume it is there if it is invisible and undetectable in any way? And how is it such that many have already made their own interpretations of the invisible and undetectable? Imaginations perhaps?
The only commonality we all have is reality. We can observe reality and agree upon many things upon which reality is based. If scientists perception of reality were as varied as theists, we would unlikely have the many things science has provided.
What has religion provided?