'Doubt' in the Bible

And immediately Jesus stretched out his hand and caught hold of him, and says to him, O thou of little faith, why didst thou doubt?

And Jesus answering said to them, Verily I say unto you, If ye have faith, and do not doubt, not only shall ye do what is done to the fig-tree, but even if ye should say to this mountain, Be thou taken away and be thou cast into the sea, it shall come to pass.