It is a sin to go against God's nature willingly, which is written on your heart. That could be interpreted as doubt I guess. But doubt is a thought not a choice, similar to a feeling. It's reactionary not a force of will, IMO. I was always raised to question everything, belief, faith, science, etc. Questioning is not a force of will either. It takes focus to question, but it isn't a decision to act. Now determining that something you believe, through the spirit, is against God's will and doing it from a selfish want, that I would call a sin.
"There ought to be a term that would designate those who actually follow the teachings of Jesus, since the word 'Christian' has been largely divorced from those teachings, and so polluted by fundamentalists that it has come to connote their polar opposite: intolerance, vindictive hatred, and bigotry." -- Philip Stater, Huffington Post
always working on cleaning my windows- me regarding Johari
always working on cleaning my windows- me regarding Johari