The Bible is a Story that bears witness to a series of events. And as a story, it has various recurring motifs, elements that stand for something else. So, the land (as in the Promised Land) stands for all of the promises of God. Mountains stand for encounters with God. Trees, and bushes, stand for God’s chosen people, and for how they respond to God and relate to the surrounding nations. The sea stands for the forces of chaos arrayed against God and the people of God.
So, when we hear the story of Moses, at the foot of a mountain, encountering God in the midst of a bush that is not consumed by God’s sheer, burning otherness being present there; and then bearing witness to the parting of the sea to let the people pass through as on dry land; we are meant to respond, “Ooh! I see...”
When Jesus speaks of exercising faith to throw mountains or trees into the sea, he is speaking about exercising faith, faithfully, (not to remove obstacles, but, rather) to claim back land from the sea, to extend the kingdom of God in the epic struggle with the kingdoms of the world ruled over by rebellious powers and principalities. To drive back chaos.
(When Jesus speaks of people being thrown into the sea, he is saying something related but different; that, if you go about contributing to the chaos, you would be better off fully identifying with chaos—from where, at least, you might be rescued by the faith of the faithful community.)
We live in chaotic times. And we get to choose. Do we bemoan the erosion of the coastline, the rising of the sea-levels? (I am speaking metaphorically here; though, what is going on in the natural world bears visible witness to what is going on at a spiritual level.) Or do we move mountains and trees? Do we proclaim God’s promises? Do we relocate ourselves in the most chaotic of communities, speaking truth to power by our witnessing presence there?