"The eye is the lamp of the body. So if your eye is healthy, your whole body will be full of light.Matthew 6:22(ISV)
The entire Mosaic Law was given to Israel through Moses, and its essence is encapsulated within the 10 commandments. Keeping the Law did not save a person but was designed to show a man his sinfulness - with the intention of driving sinners to God for His gracious salvation which would be procured through the blood of the New Covenant.
God took one small segment of all humanity (the nation of Israel) and tested them to see if they were able to keep His Law. The people of Israel were a small, test-sample of the entire, human race... but their collective failure to keep God commands placed them under the curse of the Law and demonstrated that not just they, but ALL humanity stands guilty before God as ALL are equally condemned before God... and the wages of sin is death.
But God, in His grace, determined to send a Saviour Who would identify with mankind by being born into the human race. He would keep the righteous requirement of the Law on behalf of all mankind by living a sinless life... and He would pay the price for humanities sin through His own, substitutionary death. Sin would be dealt with through the death and Resurrection of this perfect, Kinsman-Redeemer - the incarnate God, and faith in HIM would be the means to have sins forgiven, be proclaimed righteous, and receive God's great salvation.
The conditions of the Law were not set aside, but through the birth of Christ into the human race and His sinless life and ministry to the lost sheep of the house of Israel, God purposed that Jesus would fulfil every legal requirements of God's holy Law on behalf of every fallen sinner - and the single requirement to benefit from His sacrificial death and glorious Resurrection, was faith in HIM - to believe on the Lord Jesus Christ - and by believing, be given salvation as a free gift of God's grace: "Believe on the Lord Jesus Christ and you will be saved."
The amazing thing is that those who DO believe on Him are immediately removed from the devastating demands of the old Law, which He fulfilled on our behalf. And in His Sermon of the Mount, we witness Christ laying claim to be Israel's prophesied Messiah Who had come to set-up His kingdom of heaven on earth, as foretold by prophets of old. All the nation of Israel had to do to bring in the kingdom, was to repent of their sin (as a NATION), turn back to God (as a PEOPLE), and believe on the Lord Jesus Christ as MESSIAH.
The practical righteousness that Christ presented in His Sermon of the Mount was even more rigorous than the original requirements of the Mosaic Law, for righteousness was not only to be external, through the fulfilling of the Law, but also an inner virtue. Jesus used areas like prayer and fasting to teach that the sort of external perfection the Jewish rulers demonstrated, fell far short of the inner beauty of a virtuous heart that is acceptable to the Lord.
In this verse, Matthew uses the physical eyes to teach a lesson about spiritual sight. He describes the human eye as "the lamp of the body." It is through the physical eye that a man's pathway is lit and through the spiritual eye of faith that truth is seen. Our physical eyes illuminate external objects and make the features of people we meet visible - for we read that "the eye is the lamp of the body; so then if your eye is clear, your whole body will be full of light."
As His children who have been saved by grace through faith, our spiritual eyes are open. We are to manifest lives that are like a physical eye that is ablaze with the light of God's love - for the Lord Jesus is the Light of the world. He is the one Who gives us His own perfect, inner light. Those who believe on Him, are to walk in spirit and truth. They are to reflect His own shining luminosity, for He is the One that floods our inner heart with truth, and our outer life with His own beauty and the radiance of His own glory.
However, if an eye is damaged or blinded, then eyesight is impaired and vision is compromised. Instead of being flooded with the light of life, a soul is shrouded in a deep, inner darkness. Instead of seeing people and objects clearly, a man's visual perception is enveloped in a dark, inky blackness.
In this verse, Christ is teaching that those who believe in Him will receive spiritual illumination, while those that reject Him will remain spiritually blind, estranged from God, and blanketed in ungodly blackness.
The analogy of the eye being the lamp of the body, is such an easy concept that a little child can understand. How important, therefore, to keep our spiritual eyes clean and clear as we look away from that which is evil and worldly and focus our eyes on Jesus Who is our singular source of light and our sufficiency in life. When we do so, our whole body will be filled with His light and love - to the glory of God the Father.
Heavenly Father, thank You for this simple illustration of the eye being the lamp of the body. Lord, I confess that there is nothing that can commend me to You, but I praise Your name that in Christ, I have been removed from the curse of the Law and have the light of Christ shining in my heart. Help me to keep my heart pure and virtuous so that others may see the light of Christ shining in and through me, to Your praise and glory. This I ask in Jesus' name, AMEN.