And immediately Jesus stretched forth His hand, and caught him, and said unto him, O thou of little faith, wherefore didst thou doubt? -Matthew 14:31

Yea, even in Christians themselves, unbelief disables them.  A certain man had an afflicted son, possessed with an evil spirit. Jesus was up in Mount Tabor, transfigured; so the father brought his son to the disciples. What did the disciples do? They said, “Oh, we will cast him out.” They put their hands upon him, and they tried to do it; but they whispered among themselves and said, “We are afraid we shall not be able.” By-and-by the diseased man began to froth at the mouth; he foamed and scratched the earth, clasping it in his paroxysms. The demoniac spirit within him was alive. The devil was still there. … “Go!” said they; but he went not. “Away to the pit!” they cried; but he remained immoveable. The lips of unbelief cannot affright the Evil One, who might well have said, “Faith I know, Jesus I know, but who are ye? ye have no faith.” If they had faith, as a grain of mustard seed, they might have cast the devil out; but their faith was gone, and therefore they could do nothing. Look at poor Peter’s case, too. While he had faith, Peter walked on the waves of the sea. That was a splendid walk; I almost envy him treading upon the billows. Why, if Peter’s faith had continued, he might have walked across the Atlantic to America. But presently there came a billow behind him, and he said, “That will sweep me away;” and then another before, and he cried out, “That will overwhelm me;” and he thought-how could I be so presumptuous as to be walking on the top of these waves? Down goes Peter. Faith was Peter’s life-buoy; faith was Peter’s charm-it kept him up; but unbelief sent him down. Do you know that you and I, all our lifetime, will have to walk on the water? A Christian’s life is always walking on water and every wave would swallow and devour him, but faith makes him stand. The moment you cease to believe, that moment distress comes in, and down you go. Oh! wherefore dost thou doubt, then? ~ C.H. Spurgeon

http://www.blueletterbible.org/Comm/spurgeon_charles/sermons/0003.cfm