'Doubt' in the Bible

And immediately Jesus stretched forth his hand, and took hold of him, and saith unto him, O thou of little faith, wherefore didst thou doubt?

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