Doubt is not a sin.  It is a sin to refuse to believe - but the doubter is trying to believe.  It is a sin to be content in the dark - but the doubter is searching for light.  It is a sin to be indifferent about God - but the doubter wants to know God, yet doesn't have Him all figured out.

Doubt is normal.  Our limited perspective does not allow us to take in the whole picture (we just see "a poor reflection"), so it is quite natural that questions would arise.  Trying to comprehend God is like trying to fit the Pacific Ocean in a cup.  Doubt is the natural result of using limited brain cells to grasp a limitless God.