In fact, Jesus said a very small amount of faith, the size of a mustard seed, would not only move a mountain, it would also guarantee no impossibilities from that time on. But to work, the mustard seed has to be unhindered. A little bit of unbelief will nullify the power of faith. Faith was never designed to be a tug of war. When unbelief is absent, a mustard-seed amount of faith will carry a mountain into the sea.
Unbelief acts as an opponent to faith and pulls on the mountain from the other side.
Faith is nullified and the mountain remains. Jesus told the disciples that unbelief was why they were unable to cast out the devil. Although Jesus was in the same situation as the disciples, He was not moved by the attention of the crowd or the critical attitude of the scribes. He was not intimidated by the young boy who fell on the ground wallowing and foaming. He cast out the devil and set the young boy free.