John Calvin Commentary Matthew 17:19

"Then came the disciples to Jesus apart, and said, Why could not we cast it out?" — Matthew 17:19 (ASV)

Then the disciples coming. The disciples wonder that the power they once possessed has been taken from them; but they had lost it by their own fault. Christ therefore attributes this inability to their unbelief, and repeats and illustrates more fully the statement He had previously made, that nothing is impossible to faith. It is a hyperbolical mode of expression, no doubt, when He declares that faith removes trees and mountains; but the meaning amounts to this: God will never forsake us if we keep the door open for receiving His grace.

He does not mean that God will give us everything we might mention, or that may strike our minds at random. On the contrary, since nothing is more at variance with faith than the foolish and irregular desires of our flesh, it follows that those in whom faith reigns do not desire everything without discrimination, but only what the Lord promises to give. Let us therefore maintain such moderation as to desire nothing beyond what He has promised to us, and to confine our prayers within that rule which He has laid down.

But it may be objected that the disciples did not know whether or not the Lord willed to cure the lunatic. It is easy to reply that it was their own fault if they did not know, for Christ is now speaking expressly about special faith, which had its secret promptings, as the circumstances of the case required. And this is the faith of which Paul speaks (1 Corinthians 12:9). How then did it happen that the apostles were deprived of the power of the Spirit, which they had formerly exercised in working miracles, if not because they had quenched it by their indolence? But what Christ said about special faith, in reference to this particular event, may be extended to the common faith of the whole Church.