Jesus also calls us to be light. You are the light of the world. Whereas salt is hidden, light is obvious. Salt works secretly, while light works openly. Salt works from within, light from without. Salt is more the indirect influence of the gospel, while light is more its direct communication. Salt works primarily through our living, while light works primarily through what we teach and preach. Salt is largely negative. It can retard corruption, but it cannot change corruption into incorruption. Light is more positive. It not only reveals what is wrong and false but helps produce what is righteous and true.
In his introduction to the book of Acts, Luke refers to his gospel as “the first account I composed, Theophilus, about all that Jesus began to do and teach” (1:1). Christ’s work always has to do with both doing and speaking, with living and teaching.
David wrote, “For with Thee is the fountain of life; in Thy light we see light” (Ps. 36:9). “God is light,” John reminds us, “and in Him there is no darkness at all. If we say that we have fellowship with Him and yet walk in the darkness, we lie and do not practice the truth; but if we walk in the light as He Himself is in the light, we have fellowship with one another, and the blood of Jesus His Son cleanses us from all sin” (1 John 1:5–7). Light is not given simply to have but to live by. “Thy word is a lamp to my feet, and a light to my path,” the psalmist tells us (Ps. 119:105). God’s light is to walk by and to live by. In its fullest sense, God’s light is the full revelation of His Word-the written Word of Scripture and the living Word of Jesus Christ.
God’s people are to proclaim God’s light in a world engulfed in darkness, just as their Lord came “to shine upon those who sit in darkness and the shadow of death” (Luke 1:79). Christ is the true light, and we are His reflections. He is the Sun, and we are His moons. A free rendering of 2 Corinthians 4:6 could be, “God, who first ordered the light to shine in the darkness has flooded our hearts with His light. We now can enlighten men only because we can give them knowledge of the glory of God as we have seen it in the face of Jesus Christ.” God sheds His light on the world through those who have received His light through Jesus Christ.
The Jews had long claimed to have God’s light, and He had long called them to be His light. But because they had ignored and rejected His light, they could not be His light. They were confident that they were guides “to the blind, a light to those who are in darkness,” but Paul told them they were blind guides and lamps without light. “You, therefore, who teach another, do you not teach yourself?” he asks (Rom. 2:19–21). They had the light, but they were not living by it. “You who preach that one should not steal, do you steal?” Paul continues by way of illustration. “You who say that one should not commit adultery, do you commit adultery?” (vv. 21–22). We are to prove ourselves “to be blameless and innocent, children of God above reproach in the midst of a crooked and perverse generation, among whom [we are to] appear as lights in the world” (Phil. 2:15).
By its nature and by definition light must be visible in order to illuminate. Christians must be more than the largely indirect influence of salt; they must also be the direct and noticeable instruments of light.
Both in the daytime and at night, a city set on a hill cannot be hidden. It is exposed for all to see. By day its houses and buildings stand out on the landscape, and at night the many lights shining out of its windows make it impossible to miss. A secret Christian is as incongruous as a hidden light. Lights are to illuminate, not to be hidden; to be displayed, not to be covered. Christians are to be both subtle salt and conspicuous light.
God did not give the gospel of His Son to be the secret, hidden treasure of a few but to enlighten every person (John 1:9). Many reject the light and reject those who bring it, but just as God offers His light to the whole world, so must His church. It is not our gospel but God’s, and He gives it to us not only for our own sakes but the entire world’s. True believers are salt and light, and must fulfill that identity.
MacArthur New Testament Commentary