John Calvin Commentary Mark 9:20

"And they brought him unto him: and when he saw him, straightway the spirit tare him grievously; and he fell on the ground, and wallowed foaming." — Mark 9:20 (ASV)

And as soon as he saw him. That the devil should rage with more than ordinary cruelty against the man when he is brought to Christ should not cause surprise; for in proportion as the grace of Christ is seen to be nearer and acts more powerfully, Satan's fury is more intensely aroused. The presence of Christ awakens him like the sound of a trumpet. He raises as violent a storm as he can and contends with all his might.

We should be prepared beforehand with such reflections, so that our faith may not be disturbed when the approach of Christ's grace is met by more than ordinary violence from our enemy.

Nor should we lose sight of another point: that the true beginning of our healing is when our affliction is so heavy that we are almost at the point of death. It must also be considered that, through Satan's furious attack, our Lord lights a torch to make His grace visible; for when the spectators were appalled at the dreadful spectacle, the display of Christ's power, which immediately followed, was more distinctly perceived.