"Christ never failed to distinguish between doubt and unbelief. Doubt is can't believe; unbelief is won't believe. Doubt is honesty; unbelief is obstinacy. Doubt is looking for light; unbelief is content with darkness.http://www.notable-quotes.com/d/doubt_quotes.html
HENRY DRUMMOND, How to Learn How
Doubt is not a thing "shot into the soul" from without like "a loaded shell shot into a fortress." It springs up in the soul itself; and the more you attempt to "fling it away" without satisfying it, the deeper and more ineradicable it grows.
A. LAYMAN, The Suppression of Doubt Is Not Faith
Doubt comes to the door in darkness, pretending to be alone and in need of your compassionate ear. But if you let him in, he'll bring his friends."