John Calvin Commentary Mark 4:12

"that seeing they may see, and not perceive; and hearing they may hear, and not understand; lest haply they should turn again, and it should be forgiven them." — Mark 4:12 (ASV)

That seeing, they may see, and not perceive. It is sufficient to state briefly here what has already been fully explained: that the doctrine is not, strictly speaking, or by itself, or in its own nature, but incidentally, the cause of blindness. When people with weak sight come out into sunshine, their eyes become dimmer than before, and that defect is in no way attributed to the sun, but to their eyes. Similarly, when the word of God blinds and hardens the reprobate, since this takes place through their own depravity, this effect belongs truly and naturally to them, but is incidental concerning the word.

Lest at any time they should be converted. This clause points out the advantage that is gained by seeing and understanding. It is that people, having been converted to God, are restored to his favor, and, being reconciled to him, enjoy prosperity and happiness. The true purpose for which God desires that his word should be preached is to reconcile people to himself by renewing their minds and hearts. With respect to the reprobate, on the other hand, Isaiah here declares that the stony hardness remains in them, so that they do not obtain mercy, and that the word fails to produce its effect on them, so as to soften their minds to repentance.