John Calvin Commentary Matthew 6:23

"But if thine eye be evil, thy whole body shall be full of darkness. If therefore the light that is in thee be darkness, how great is the darkness!" — Matthew 6:23 (ASV)

If the light which is in thee be darkness. Light signifies that small portion of reason which continues to exist in men since the fall of Adam; and darkness signifies gross and brutal affections. The meaning is, we should not wonder if men wallow so disgracefully, like beasts, in the filth of vices, for they have no reason which might restrain the blind and dark lusts of the flesh.

The light is said to be turned into darkness, not only when men permit the wicked lusts of the flesh to overwhelm the judgment of their reason but also when they give up their minds to wicked thoughts and thus degenerate into beasts.

For we see how wickedly men change into craft any measure of wisdom which had been given to them: how they “dig deep (as the prophet says) to hide their counsel from the Lords” (Isaiah 29:15), how they trust to their own resources and openly dishonor God—in a word, how desirous they are to show their ingenuity in innumerable ways for their own destruction.

Christ has good grounds for declaring that thick and appalling darkness must of necessity reign in the life of men when they choose to be blind.

This is also the meaning of the words found in the Gospel of Luke, with this difference: Christ there connects the present statement with one previously explained, that men do not light a candle and put it under a bushel (Matthew 5:15). Again, instead of the clause if the light which is in thee be darkness, He gives the exhortation: see that the light which is in thee be not darkness.

The meaning is, “See that your mind, which should have shone like a candle to guide all your actions, does not darken and mislead your whole life.”

He afterward adds that when the body is enlightened by the eye, the greatest regularity is found in all its members, just as the light of a candle spreads and penetrates into every part of the room.