The eye is the lamp of the body. So, if your eye is healthy, your whole body will be full of light, but if your eye is bad, your whole body will be full of darkness, how great is the darkness!
If you are absorbed with money, you will miss everything else in life that really matters.
James Montgomery Boice
It is because of the eye that a man is able to make use of the light. Therefore, in this secondary sense, the eye may itself also be called the body’s light or lantern.
It is a well-known fact that lack of sufficient light from sun, moon, stars, lamps, etc., makes it difficult to see things. Yet, a sound eye quickly adjusts to the darkness. But if the eye itself, the very organ of light, is in poor condition, the darkness will be great indeed. In that case, even if the sun were shining, not much would be gained. At best, everything would be indistinct, a huge blur.
Implication on the basis of verses 19-21: Just as a person has a natural eye…to illumine his physical existence and to bring him into contact with his earthly environment, so he has a spiritual eye, namely, the mind, to brighten his inner life, to guide him morally and spiritually, an to keep him in contact with the heavenly Father. But if the “light” that is in him be darkened—for example, by means of his inordinate yearning for earthly treasure–, then how great must be that darkness, the very organ of light-reception having been obscured by sin. By missing what should have been his goal, namely, the promotion of God’s glory, this person misses everything!
God must be king over our hearts: his law, his will, his precepts must receive our first attention; then, and not till then, everything in our inner being will fall into its right place. Unless our hearts are set in order like this, everything will be confusion. “Your whole body will be full of darkness”
If our faith is presumption, our zeal selfishness, our prayer formality, our hope a delusion, our experience infatuation, the darkness is so great that even our Lord holds up his hands in astonishment and says—‘How great is that darkness!’
Oh, for a single eye to God’s glory, a sincere consecration unto the Lord! This alone can fill my soul with light.