Doubt is not Unbelief
Yes, we know doubt is a sin, but how do we identify this sin which seems to have full dominion over us? From a biblical perspective we must be careful in defining doubt, Doubt is not unbelief, unbelief is a lack of faith, the mindset of someone that does not want to trust God. So unbelief is an unwillingness to trust God while doubt is a weakness in trusting God. The two are not opposites of one another but they are surely not the same as one another. I define doubt this way.
No One is Above Doubt
Consider the Prophet Elijah;
Bruised Reed and Dimly Burning Wick