I think this will throw some light upon the words of our Lord, “If ye have faith and doubt not, if ye shall say unto this mountain, Be thou removed, and be thou cast into the sea; it shall be done.”

Good people, among them John Bunyan, have been tempted to tempt the Lord their God upon the strength of this saying, just as Satan sought to tempt our Lord on the strength of the passage he quoted from the Psalms. They think that as long as they have faith, and believe earnestly enough, it is possible to do and accomplish anything to which they might set their hand.

Happily for such, the assurance to which they would give the name of faith generally fails them in time. Faith is not the fervent setting of the mind on “believing” for such-and-such an outcome — more often than not a desire generated by the man’s own soul — as if we, and not God, were the orignators and initiators of faith by the strength of our passions, the fervor of our prayers, and the forcefulness of our mental processes. True faith, rather, is that which, knowing the Lord’s will, goes and does it, or, not knowing it, stands and waits, content in ignorance as in knowledge, because God wills. Faith neither presses into the hidden future, nor is careless of the knowledge that opens the path of action. It is faith’s noblest exercise to act with uncertainty of the result when the duty of obedience is certain, or even when a course seems with strong probability to be duty. Even if a man is mistaken in the honest effort to obey, though his work be burned, by that very fire he will be saved. Nothing saves a man more than the burning of his work, except the doing of work that can stand the fire.

But to put God to the question in any other way than by saying “What will you have me to do?” is an attempt to compel God to declare himself, or to hasten his work, or to imagine it his work what our own soul desires to accomplish.

— George MacDonald, Knowing the Heart of God, p. 274-275