The passage we studied yesterday, Matthew 6:19-21, focused on "treasures" (i.e., whatever is most important to one's "heart"). It is foolish to store up earthly treasures for they do not last, but heavenly treasures are eternal. Matthew 6:24 also addresses this theme. One cannot serve earthly riches and God simultaneously. There can only be one master in a person's life. If you make physical riches your master, then you have chosen a treasure that will not last. If you make God Almighty your master, then you have wisely chosen an everlasting treasure.
Attempting to study Matthew 6:22,23 out of context would be challenging. What does Jesus mean when He speaks of good eyes, bad eyes, darkness, and light? However, understanding His point is not nearly as difficult when one observes that Jesus is contrasting earthly and heavenly treasures in the verses both immediately before and after Matthew 6:22,23. Thus, it is reasonable to suggest that He is still addressing this theme in these verses in the middle.
"The lamp of the body is the eye" (6:22). The eye allows images (light) inside the body. One's body will be "full of light" if his eye is "good" (i.e., healthy) and allows him to see things clearly and in proper perspective. One with such good spiritual "eyesight" sees money as a tool to help further the Lord's work and not as something to lavish upon himself.
But, if one's eye is "bad" (i.e., not healthy), then his body will be "full of darkness." He will not see things clearly or in a proper perspective, physically or spiritually. It is entirely possible for one to start out with a healthy "eye," but it can become dimmer and dimmer until it is full of the evils of materialism and immorality.
"No one can serve two masters" (6:24). It should be remembered that Jehovah is a jealous God (Exo. 20:5), and just as no right-thinking husband will accept a rival for the affections of his wife, so God will accept no rival for our affections. If a person loves anyone or anything more than he loves God, then that person is not a true disciple (Luke 14:26-33); he is serving another "master." Also it should be recognized that a person does not necessarily love what or whom he claims; he loves that to which he gives foremost devotion and attention.
Jesus contrasts "God" with "mammon" (i.e., riches). It is impossible to serve these two masters simultaneously because the only way to obey the one is to disobey the other! If a man loves and treasures physical things, then he will not serve God with all his heart. God will not accept just part of his service but demands that he love Him supremely (cf. James 4:4; I John 2:15). However, the devil will gladly accept only a portion of man's service, for he knows that in getting a part of it, he is truly getting all of it.
The primary thrust here seems to be pointing out the danger of self-deception. A person might falsely reason that he can serve riches as long as he is serving God by means of certain formalities. Jesus says that such is not possible. You're either with God or against Him; there is no middle ground (Matt. 12:30)! Friends, where is your treasure? If you answer that question honestly, you'll know who your master is!