"If your eye is sincere..."
To know the great truths of God requires that we dispute the distractions and problems of the world. To seek God with one foot in the world essentially corrupts our mission, and we end up with a false deity. It is necessary that we see Jesus for who He is, and not who we decide Him to be. Jesus reminds his listeners of the focus needed by the Kingdom citizen, and the benefits of that focus. We would all do well to heed this great lesson.
Our "eyes" (in a physical as well as spiritual sense) are the windows we use to perceive the world. They truly are the "lamp[s] of the body". Our eyes are the only way that light enters our bodies at all. In a spiritual sense, we must consider that our "eyes" perceive external things. Jesus is not indicating a "found truth" within ourselves, but implying an external stimuli reaching inward. Any idea that we create truth within ourselves is exposed as faulty by the very example that Jesus uses. We know that our eyes do not create light; rather, they process light in ways that our brains interpret accordingly. Thus, our spiritual "eyes" are meant to receive the truth of God and transmit it inward to our hearts. We have to desire this, just as some Gentiles desired to "see Jesus" (John 12:21).
This spiritual "sight" depends upon how our eyesight is trained and focused. One who has vision problems requires glasses or contact lenses to see, or perhaps undergoes laser eye surgery to make the necessary corrections. In much the same way, we must be taught how to "see" Jesus properly. This requires our eye to be "single" (KJV), or "sincere" (NKJV). This focus should pervade everything we do as a disciple of the Messiah. It must overwhelm every decision we make, and everything we do or say. Paul understood this: "And whatever you do in word or deed, do all in the name of the Lord Jesus, giving thanks to God the Father through Him." (Colossians 3:17).
Finally, we must remember the consequences of a lack of focus: deception and condemnation. Our perception of truth can negatively affect everything we are. The obvious problem occurs when we become consumed with the things of the world (politics, squabbling, crises, suffering) and our focus goes away from the LORD. Something much more sinister is when we are "following God" in name, but in our focus looking at everything else but Him. Israel had this same problem (Isaiah 55:1-9). If our focus is lacking in this way, we will find nothing in the end but total darkness (Matthew 8:12; 22:13; 25:30).
God gives us a way to go. "You will make known to me the path of life. In your presence is fullness of joy. At your right hand are pleasures forever." (Psalm 16:11). He does not hide Himself, but makes Himself available to all. "And I will not hide My face from them anymore; for I shall have poured out My Spirit on the house of Israel,’ says the Lord GOD.” (Ezekiel 39:29). He invites us all to see Him as He is. "Beloved, now we are children of God; and it has not yet been revealed what we shall be, but we know that when He is revealed, we shall be like Him, for we shall see Him as He is." (1 John 3:2). "And the Spirit and the bride say, “Come!” And let him who hears say, “Come!” And let him who thirsts come. Whoever desires, let him take the water of life freely." (Revelation 22:17)
If we want to see God, we can. He tells us how to do so. Praise God for his openness and genuine lovingkindness! –Steven McCrary