But withering a fig tree is not, in this case, mechanical. The operation on it supersedes natural means. I could probably spray some chemical on a tree and wither it rather quickly, but just speaking to it would have no effect. But, is that because I just don't believe it will have an effect, and therefore I doubt? There is no apparent relationship between speaking a word and the result. This fact produces doubt. Jesus tapped into a supernatural mechanism that we humans cannot understand. No engineer can create it. No scientist can discover it because it is not of this world.
But is Jesus saying that one function of faith, which is essential to our relationship with Him, is to throw mountains into the sea? It is like these icons on my computer that I never click on because I do not understand what they do. So I am afraid what might happen. They are there and they will do something if I click on them, but not until. It is so difficult to grasp miracles, and how a common man can initiate one.
One thing seems to be clear in Jesus' words, i.e. there is much more possible to us as followers of the Son of God than we often see in our experience. It is either because we have not cultivated "faith without doubt" or God simply does not need us to do all that is possible, and limits us for practical reasons. Think of the chaos if we were randomly throwing mountains into the sea!