Matthew 21:21

‘And Jesus answered and said to them, “Truly I say to you, If you have faith, and do not doubt, you will not only do what is done to the fig tree, but even if you shall say to this mountain, ‘Be you taken up and cast into the sea,' it will be done.”

Jesus replies enigmatically. He primarily uses what He has done as an illustration of what true faith can do, and even expands on it. He will leave the deeper lesson to be understood later. So He points out that nothing is impossible to faith, even the withering of fig trees and the moving of mountains and casting of them into the sea. However, we must not read into that that faith can produce anything that we wish (it did not produce figs for Jesus to eat), for it would be no more moral for us to use faith for our own selfish purposes as it would have been for Jesus. The point is that we can only use faith in this way if there are grounds for such faith. Jesus is not saying to His disciples that they can do anything ridiculous that they decide that they want to do (like moving a mountain simply in order to avoid having to climb over it). He is saying that this is true for anything that they have good grounds for thinking is in the will of God. Indeed He may well have intended them to remember the mountain moved by Zerubbabel (Zechariah 4:7), that is, the achieving of seemingly impossible spiritual objectives because inspired and empowered by the Spirit.

But He then goes on to add a further spiritual lesson. For ‘this mountain' must mean either the Temple mount, or the Mount of Olives, or the mountain on which Jerusalem was built, probably the first (this is clearer in Mark), while being cast into the sea regularly elsewhere indicates judgment (Matthew 8:32; Matthew 18:6; Mark 9:42; Luke 17:2). Thus He is not only indicating the future fate of old unbelieving Israel, but also the future fate of Jerusalem, both of which are coming, and both of which will take place because of the prayers of faith of the disciples, not so much as a result of praying for such results specifically, but because their prayers for the establishment of the new congregation will inevitably result in it (e.g. Acts 4:29). An Israel denuded of believers will be a withered Israel indeed.