John Lightfoot on Matthew 21:21

[But if ye shall say unto this mountain, Be thou removed, and be thou cast into the sea; it shall be done.] this is a hyperbolical way of speaking, taken from the common language of the schools of the Jews, and designed after a manner for their refutation. Such a hyperbole concerning this very mountain you have Zechariah 14:4.

The Jews used to set out those teachers among them, that were more eminent for the profoundness of their learning, or the splendour of their virtues, by such expressions as this; He is a rooter up (or a remover) of mountains. "Rabh Joseph is Sinai, and Rabbah is a rooter up of mountains." The Gloss; "They called Rabh Joseph Sinai, because he was very skilful in clearing of difficulties; and Rabbah Bar Nachmani, A rooter up of mountains, because he had a piercing judgment." "Rabba said, I am like Ben Azzai in the streets of Tiberias." The Gloss; "Like Ben Azzai, who taught profoundly in the streets of Tiberias; nor was there in his days such another rooter up of mountains as he." "He saw Resh Lachish in the school, as if he were plucking up mountains and grinding them one upon another."

The same expression with which they sillily and flatteringly extolled the learning and virtue of their men, Christ deservedly useth to set forth the power of faith, as able to do all things, Mark 9:23.