Three times Jesus uses the nature of the mustard seed to illustrate that something, though small as the mustard seed, can grow into something large. In Mt. 13.31-32 Jesus speaks of the mystery form of the Kingdom of Heaven, the church, starting small and becoming large in time.
Two other times the mustard seed is featured when Jesus teaches that faith can accomplish extraordinary deeds. Luke 17.5,6 has the disciples asking Jesus to increase their faith with His reply saying that if they had the faith of a mustard seed they could say to a tree “be uprooted and cast into the sea, and it would obey you.” A second time the same lesson of faith regarding a the mustard seed occurred after Jesus came down from the mount of transfiguration where He healed a demon possessed boy which the disciples had failed to cast out. Afterward, the disciples asked why they had failed with Jesus stating that their faith was too small. He then says that if they had mustard seed faith, they could command the mountain to remove and it would and additionally, nothing would be impossible for them.
Some commentators see the promise as rhetorical hyperbole indicating faith in God accomplishes great things. However, even if the promise of mustard seed faith is literal, not even the Christian Apostles performed the miracle. So it is safe to say that this promise is not something to be performed on a routine manner. Additionally, not even Jesus performed it, though undoubtedly, He could have. It did not figure into the signs He gave to authenticate Himself or was it a sign given during the time of great expansion of the church in the early centuries. So, Christians should strive to have the faith of a mustard seed undoubtedly but perhaps they should first aim for a lesser miracle before attempting to move mountains literally.