'Doubt' in the Bible

At once Jesus reached out his hand, caught him, and asked him, "You who have so little faith, why did you doubt?"

Jesus answered them, "I tell all of you with certainty, if you have faith and do not doubt, not only will you be able to do what has been done to the fig tree, but you will also say to this mountain, "Be removed and thrown into the sea,' and it will happen.