Jamieson, Fausset and Brown on Mark 11:23

23. For verily I say unto you, That whosoever shall say unto this mountain, Be thou removed ... he shall have whatsoever he saith--Here is the lesson now. From the nature of the case supposed--that they might wish a mountain removed and cast into the sea, a thing far removed from anything which they could be thought actually to desire--it is plain that not physical but moral obstacles to the progress of His kingdom were in the Redeemer's view, and that what He designed to teach was the great lesson, that no obstacle should be able to stand before a confiding faith in God.