Doubt is the fuel for Faith. It is also a marvelous pillow, for a well constructed head.
"Only that God who sees in highest heaven: he only knows whence comes this Universe, and whether it was made or un-created. He only knows . . . Or perhaps he knows not." - Rig Veda X, 129
Unfortunately, "doubt is the fuel for faith" becomes hideously comical when seen in the context of modern Christianity. The Christianity of today sets up faith as a kind of challenge - to make life more interesting. Doubt is essential, because without doubt there would be no such challenge, and no fun. Consequently, Christianity has gone out of its way to believe in the most ridiculous notions humanly possible, to ensure doubt for all but the most brainless. Indeed, people are never happy unless they're never happy.
People these days cultivate doubt because it is an intellectual status symbol. Thus their doubt is an imitation doubt. And because they never truly doubt, they never arrive at a real resolution to life's problems. They doubt everything except that which should be doubted - the heart, which after all, is unreliable.
True doubt is a kind of true faith, because it is a faith in reason, which is indirectly a faith in truth.