Mark 9:23 Commentary - Whedon's Commentary on the Old and New Testaments

23. If thou canst believe An echo of the man’s expression, If thou canst do anything. The man had evidence which required him to have and to use a proper amount of faith. As God does not require our first faith without giving us a first evidence, so our Lord first gave prior evidence of his divinity in order to create a first faith. But when that was done, the condition of the exercise of faith was an inexorable demand. Our Lord thus performed, as we may say, two classes of miracles.

All things are possible to him that believeth When our Lord says “all things,” we are to understand what classes of things he is speaking of, in which he includes all. And the condition (“to him that believeth”) belongs not to every rash and presumptuous belief, that the mind, not in communion with God, may conjure up. The belief and the grant to prayer of which Jesus speaks belong perhaps to the world in which he speaks, namely, the religious and spiritual world. And the belief of which he speaks is that faith of which God grants the power. All things within its sphere are possible to that faith; for God will not grant power to faith for things which he will not make possible.