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. If then the light in you is darkness, how great is the darkness!
Matthew 6:22-23
The lamp of the body is its gaze; with clear vision, your whole being will shine brightly.
We have clear vision when we have a single purpose. There is nothing that can distract us from our primary objective. A person who is consistent and true stands out from a crowd because they have a definitive purpose and role. Standing out creates a separate-ness, a holiness, that can positively influence others.
Interestingly, Yeshua here contrasts the person who is single-minded with someone whose eye is considered “bad” or “wicked.” When we don’t have clear purpose, we tend to have divided interests among many other things that may not be in our best interest and lead us into wickedness. These secondary objectives cloud our vision and create a darkness that envelops our judgment.
The larger context of this saying is that it is joined directly to Yeshua’s admonition that we cannot serve two masters: God and worldly wealth. In this respect, focusing primarily on worldly gain will divide up our interests more, taking us further from singleness of purpose.
When we constantly look upon the things of God and his kingdom, our lives of unified purpose and godly intentionality become shining examples to others.
The Core of the Bible 