John Calvin Commentary Mark 9:21

"And he asked his father, How long time is it since this hath come unto him? And he said, From a child." — Mark 9:21 (ASV)

From a child. From this, we infer that this punishment was not inflicted on account of the sins of the individual, but was a secret judgment of God. True indeed, even infants, as soon as they have come out of the womb, are not innocent in the sight of God, or free from guilt; but God’s chastisements sometimes have hidden causes and are intended to test our obedience. We do not render to God the honor which is due to Him, unless with reverence and modesty we adore His justice, when it is concealed from us. Whoever wishes to obtain fuller information on this point may consult my Commentary on these words, Neither hath this man sinned, nor his parents (John 9:3).