John Calvin Commentary Mark 9:23

"And Jesus said unto him, If thou canst! All things are possible to him that believeth." — Mark 9:23 (ASV)

If you can believe. “You ask me,” he says, “to aid you as far as I can; but you will find in me an inexhaustible fountain of power, provided that the faith which you bring is sufficiently large.” From this, a useful doctrine can be learned which applies equally to all of us: it is not the Lord who prevents His benefits from flowing to us in large abundance, but it must be attributed to the narrowness of our faith that it comes to us only in drops, and frequently we do not even feel a drop, because unbelief closes our heart. It is an idle exercise of ingenuity to prove that Christ’s meaning was that a man can believe by himself, for nothing more was intended than to throw the blame for their poverty back on men whenever they disparage the power of God by their unbelief.

All things are possible to him who believes. Christ undoubtedly intended to teach that the fullness of all blessings has been given to us by the Father, and that every kind of assistance must be expected from Him alone in the same manner as we expect it from the hand of God. “Only exercise,” he says, “a firm belief, and you will obtain.” How faith obtains anything for us we will immediately see.