Suppose there is someone who does not doubt in his heart (Mark 11:23)–that is to say, who does not dispute in his intellect–and through such doubt sever that immediate union with God which has been brought about by faith, but who is dispassionate or, rather, has already become god through union with God by faith: then it is quite natural that if such a person says to a mountain, ‘Go to another place’, it will go (Matthew 17:20). The mountain here indicates the will and the law of the flesh, which is ponderous and hard to shift, and in fact, so far as our natural powers are concerned, is totally immovable and unshakable.
~St Maximos the Confessor
Prayerful Life 