19 And when he saw a fig tree in the way, he came to it, and found nothing thereon, but leaves only, and said unto it, Let no fruit grow on thee henceforward for ever. And presently the fig tree withered away.
20 And when the disciples saw it, they marvelled, saying, How soon is the fig tree withered away!
21 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.
22 And all things, whatsoever ye shall ask in prayer, believing, ye shall receive.
The fig tree produces fruit before leaves, thus since the tree was producing leaves it also produced fruit. The tree is representing the Jewish religious system at the time which appeared healthy (leaves on the tree) but was producing no fruit. This event occurs after the cleansing of the temple and Jesus had declared the house of prayer as a den of thieves; he is now passing judgement on a barren Israel.
The instant withering of the tree after the curse metaphorically pronounces the decline of Judaism. The essence of the curse also indicates the upcoming shift in the work of God. Matthew wrote this about 80 AD and could also be referring indirectly to the destruction of the temple in 70 AD. So, not only was this spiritually effective, but people saw the real downfall of the temple, Judaism's symbol of faith.
Faith is not a magical power that we can use on our own, but we need to use it in conjunction with prayer. Prayer does not activate faith, but faith must be proffered to God in prayer. Thus, it is God's will that determines the movement of the mountain.
Jesus' response to his disciples that they should have faith and not doubt thus having power thru that faith indicates that their roles in God's Kingdom as servants to the people of the world in the dawning of Christ. But they should not utilize the power of God to fulfill their wants or needs or perform miracles they want to perform, Jesus had just condemned those actions. They are to use their power as God's people to produce the "fruit" that Israel could not produce, in other words be fruitful.