What does Matthew 6:23 mean?

In this part of the Sermon on the Mount (Matthew 5:1–2), Jesus is using sight and blindness—light and darkness—as a metaphor to illustrate his teaching on stockpiling treasure in heaven. In keeping with the theme of chapter 5, this involves making right choices out of sincere devotion to God. In this specific teaching, He has warned against storing up treasure on earth, rather than treasure in heaven. Unhealthy reliance on worldly wealth reflects greed; God, not the world, ultimately provides what we need (Matthew 6:19–21).

Jesus has just noted that the eye is the lamp of the body (Matthew 6:22). A healthy eye perceives light and transmits that information to the body. In a poetic sense, a functioning eye floods the body with light. Light allows a person to see and operate in the world according to what is true (Proverbs 4:18–19; Matthew 4:16; 5:13; John 8:12; 2 Corinthians 4:6).

Here, He adds the counterpoint to His metaphor. An eye that is bad—one that is blind or injured—lets no light into the body. This is true both spiritually and physically. The inner part of the person remains cloaked in darkness, and it is a powerful darkness.

What makes the "spiritual" eye blind or unhealthy? In the context of Jesus' metaphor, serving money instead of serving God causes a person's spiritual eyes to be blind. That wrong priority is like a cataract that blocks light—it's a spiritual flaw that interferes with truth. This results in inner darkness. Serving God by making right choices and storing up "treasure" or rewards in heaven is like having a good eye, flooding a person's inner world with the light of God's righteousness.