There is a popular way, in this day & age, to speak of doing things that may seem impossible for the Average Joe, but that's not really what Jesus had in mind when He spoke in this way to His disciples in the first century! To His disciples in that generation, Jesus promised that if they held tight to Him, they would not be lost when the 'mountain' in question was cast into the sea. In some small sense, maybe, this enigmatic phrase may be borrowed for today's vernacular, in that, with Faith, we can do the impossible, but we must remember that though our Faith may be the same Firm Foundation as those first disciple's, we do not face the 'mountain' they did; their 'mountain' was one which led to a precipice & whose removal changed the face of Worship forever!
When Jesus spoke those fateful yet comforting words to His disciples, He told them, as He told the Woman at the Well that the 'mountain' that they had been brought up on, 'Mount Zion on the sides of the north' ( Psalm 48:2 ) was about to be removed as the place of worship; instead, they would now worship 'in spirit and truth'. The Faith that He spoke of, however, though they would employ it, was not theirs, per se', but was the Power of Eternal Life which resided in Him alone! Mere belief, or even trust & hope, would not do it; His Faithfulness alone, through their perseverance & sufferings would accomplish this salvation!
Certain people will argue till the cows come home ( or they're blue in the face-whichever comes first ) about whether the Faith described in Scripture is ours or Christ's ( God's ); some say that the apostle Paul in his letter to the Church at Ephesus meant to say that Faith is a Gift of God, others that he is simply saying Salvation is a Gift of God, but I say that it is both: actually, we can bypass both arguments because this letter was to those first-century saints at Ephesus, not to us & because the salvation referred to was salvation from the Death of Adam under that first covenant, we are free from that Law! However, we may truly say ( Scriptures like II Timothy 2:13 attest to this ) that it is the Faith of the Creator God, as embodied in the Messiah of Israel, that saved His People from that Death!
The old hymn goes, 'My faith has found a resting place'; this is true enough, as far as that goes & while we may truly say that our own belief, the trust which we have placed in the Creator of Heaven & Earth is necessary, it is ultimately His Faithfulness that shines through! James the brother of our Lord wrote, 'faith without works is dead' ( James 2:20 ): speaking to a brother the other day about the passage from Matthew's Gospel account & the popular interpretation of it, I reminded him of this passage; 'men can literally move mountains in this day & age; we have the tools & technology to actually uproot a mountain and move it to another spot, though it will take time & hard work!' Jesus', I'm pretty sure, wasn't ensuring His disciples that they would be able to actually move literal mountains, although I won't deny that's possible; He was telling them, in a metaphor, that the 'mountain' that had held sway over them for so long would finally come crashing down & be swallowed by the 'sea'!
We must exercise the Faith that we hold, but as the apostle told Timothy 'If we are faithless,
He remains faithful; He cannot deny Himself' ( II Timothy 2:13 ) it is HIS faith that stands strong, that moves mountains! Our personal belief/trust/faith/hope may waver & we will fail & fall, but the Faith of our Father God & Creator stands strong & will never fail. It is His Faithfulness that extends a mighty Hand & raises us up from the ash heap, as He rose Israel ( Jesus ) from the Grave & gives us New Life, New Purpose, Resurrection!
Jesus promised His disciples that 'they' would move mountains! Did they? Surely; by preaching the Good News & by persevering even to the Death of the Body, the disciples of Jesus turned the world upside down & caused the 'mountains' of that day to be engulfed by the waters of the sea! Was it their faith that removed that 'mountain' & cast it into the sea? It most assuredly was through their faith in action that the promises of God were fulfilled, but primarily, it was His Faithfulness, in the Person of Jesus the Christ that definitively won the battle!
Charles Haddon Shank