He pointed out that the verses preceding this famous line concern Jesus telling His disciples to forgive those who wrong them. The disciples then ask to have their faith increased. It is here that the line comes up. He is telling them that they have the faith needed to accomplish their tasks, but that they need to be willing to use that faith. In a sense, they need to have faith in their faith.
I'm reminded of the repeated line in "Back to the Future": "If you put your mind to it, you can accomplish anything." We so often lack the confidence necessary to take the steps that we know we need to take. That confidence is there, waiting for us to use it.
This does not mean that the way will be easy or that we should expect rewards for our determination. Rather, the way will likely be difficult and the rewards few. However, with faith we move forward, not frozen in indecision and fear. We have the faith to move mountains available to us; we simply need to use it to move ourselves.