As far as I'm concerned, it's the doubt that proves if it's applicable or not.
For exsample: Someone says s/he belives in the bible. But NEVER actualy read it or took her/his time to see if it's the right thing for the person.
Doubt would help to sort it out for good. Means people that have proven thier Faith / Believe trough doubt, are more firm in their believes and principles, to act accordingly.
I've never met a scientist that discarded the notion of some creational power right away.
But I met alot of "fundametalistic believers" of alot of "religions", that would discard anything for their believe.
(Just in case the question arises, it where mainly Christians and Muslims, that would rather burn me, then to think about what I said ... But I had the same trouble with somekind of Hindi and a Satanist

)
So, doubt is, in my eyes, the main pillar of everything you might choose to belive in.
Only after proven that way -wich is a pure personal epiphany- someone might decide to believe in something 100%.
That onto itself would fall in the free will department
And the free will as such is something to discuss elsewhere ...
