Christ's "If" answered and more than answered the man's "if." The man had said, "if thou canst do anything"; Christ reversed it and showed where the real contingency lay. "If thou canst believe, all things are possible to him that believeth." And why are all things possible to him that believeth? Because he that believeth takes hold of Christ and uses His omnipotence.
I. Observe first the expression. "If thou canstbelieve," not, "if thou dostbelieve." Every man who has not made himself lower than a man, and so lost the position of our common humanity every man has some faith. And every man who uses the faith he has, will increase its' power and acquire more. You are the arbiter of your own creed, and your faith is also the test of your own moral condition. And our Lord was not mocking the father of the lunatic child He was not making a requirement of that which was an impossibility, but He was elevating his mind, and carrying on his own spiritual life, when He said to him "If thou canst believe."
II. The outside boundary line of the province of faith, properly so called, is promises. Faith is laying hold, I do not say of what God is, for God may be and is much which we cannot understand enough even to believe but it is laying hold of what God has covenanted Himself to us what God is to His people. The promises are what God is to His Church, therefore faith confines itself to promises.
III. The text does not say "All things are given to him that believeth," but "All things are possible to him that believeth." It may happen, for various causes, that a man may not, at a certain period, receive even what he believes and seeks. God may have some wise, secret reason for not giving it at that time. The man himself, though, he has the faith, may yet have to learn how to use and express his faith better. There is no promise respecting the time, or the way; there is the promise, but not the how or the when. All that is asserted is this, that when a man has the faith of a mercy, he has then the possibility of that mercy. Then, all barriers have been removed, and he may have that mercy at any time, and be sure to have that mercy some time.
J. Vaughan, Sermons,1868, p. 85.
References: Mark 9:25. Spurgeon, Sermons,vol. viii., No. 474; vol. xxix., No. 1744; Ibid., Evening by Evening,pp. 222, 281; J. M. Neale, Sermons for the Christian Year,vol. ii., p. 193.