“Jesus answered and said unto them, Verily I say unto you, If ye have faith, and doubt not, ye shall not only do this which is done to the fig tree, but also if ye shall say unto this mountain, Be thou removed, and be thou cast into the sea; it shall be done.”
And Jesus answered them: truly, I say to you, if you have faith and do not doubt, you will not only do what has been done to the fig tree, but even if you say to this mountain, be taken up and cast into the sea, it will happen. The faith without doubt that moves mountains is the teaching that Matthew 17:20 established (mustard-seed faith moves mountains) now applied to the disciples who witnessed the fig tree. The mountain that can be cast into the sea is the mountain before them — possibly the Mount of Olives, whose overthrow was associated with the messianic age in Zechariah 14:4.
Church Fathers on Matthew 21:21
A bad man is better overcome by giving way to him than by replying to him; for wickedness is not instructed but stimulated by reproof. The Lord accordingly sought by withdrawing Himself to check those whom His words could not check; whence it is said, 'And He left them, and went out of the city into Bethany.'
Hence it is to be understood that the Lord was in so great poverty, and so far from having courted any one, that He had found in all that city neither entertainer, nor abode, but He made His home in a little village, in the house of Lazarus and his sisters; for their village was Bethany; and it follows, 'and He lodged there.'
For in permitting His flesh to suffer that which properly pertains to flesh, He foreshews His passion. Mark the earnest zeal of the active labourer, Who is said to have gone early into the city to preach, and to gain some to His Father.