Doubt is natural within faith. It comes because of our human weakness and frailty…. Unbelief is the decision to live you life as if there is no God. It is a deliberate decision to reject Jesus Christ and all that he stands for. But doubt is something quite different. Doubt arises within the context of faith. It is wistful longing to be sure of the things in which we trust. But it is not and need not be a problem.
Alistar McGrath as quoted in Disciplines of a Godly Family by Kent and Barbara Hughes