Light also is a positive image: the world is in darkness and Jesus’s followers are to be a light in that darkness. A “light in the darkness” is part of the messianic age, the suffering servant was to be a “light in the darkness.” Zechariah alludes to this in Luke 1:79, his son John will shine a light for those living in the darkness. They are in the dark because they live “in the shadow of death.” The Gospel of John describes Jesus as the “true light that gives light to everyone” (John 1:9) which overcomes the darkness (1:4-5).
Jesus offers two metaphors for the way his disciples were to be light in the world, a city and a lamp. First, a city on a hill cannot be hidden. it can be seen from a distance and any light from that city will be seen clearly in the darkness. Jeremias (Parables, 217) understands the saying as a word of comfort for the disciples, the “citizens of the … eschatological city of God … whose light streams through the night needing no human efforts.” Second, when a lamp lit, it was normally placed on a stand or in a niche in the wall so the light can illuminate the whole room. In makes no sense to hide an oil lamp under a basket, the point of a lamp is to shine light in the darkness.
The point of the two illustrations is that it is impossible for the Christ-follower to hide their light, and even if they could manage to hide their light, it makes no sense to do so since their entire purpose is “being a light.”
If the “Salt of the Land” referred to the disciples as a preserving agent within Judaism (as opposed to the Pharisees), then “Light of the World” refers to the function of Israel as the light to the Gentiles. Jesus uses κόσμος rather than γῆ, so the whole world (Jew and Gentile) is in view rather than just the Land of Israel.
Pennington points out that Scot McKnight is unique in his assertion the two words refer to different things. He argues they are in parallelism and therefore have the same reference. Pennington, Sermon on the Mount, 164, note 74. Late he says “both the salt and light metaphors are communicating the same idea, that Jesus’s disciples are not the heralds of the new and lasting covenant being effected by Jesus” (p. 165). While it is true Jesus’s disciples are heralds of the new covenant, is that what this passage is actually saying? He argues that salt and light overlap in the area of covenant, salt used with bread to conclude a covenant agreement, and light as associated with the messianic age of the new covenant.
Israel was supposed be so devoted to God, living out a wise lifestyle, that the nations would see them and be attracted to God. They were the “light of the world” in the Old Testament, but they failed to be devoted to the Law and failed to live out a wise life and rarely attracted Gentiles to the God of Israel. There are some examples of Gentiles honoring the God of Israel (Nebuchadnezzar, for example, perhaps Nehemiah, Esther and Mordecai), but for the most part Israel did not act as the “light to the world.” Ultimately, the “light to the whole world” is the messiah, especially in Isaiah 9:2, a passage quoted by Matthew 4:16 (cf., Isaiah 42:6, 9).
Finally, the followers of Jesus are to let their light shine in the world so that people will see this and glorify the Father. This too was supposed to be a function of Israel in the Old Covenant (Deut 4:5-8). If Israel is obedient to the Law, then the nations will see this and consider Israel to be a great and wise nation.
The followers of Jesus are to be the preserving agent in their culture; they are still the only light in this dark world, the only want that light can be seen is if it is active in the world in some real and tangible way. Good works is “a translation of the Jewish מַעֲשִׂים טֹובִים, the thought is of those demands of God that are not legally prescribed by the Torah, such as especially works of charity and almsgiving” (Luz, Matthew 1-7, 208).
There are many examples of Christians who live out their faith in a way which benefits the whole culture and demonstrate to the culture the light of Jesus Christ. Some Christians live out a Christ-like lifestyle in a way which makes life better, healthy, etc. so that people are attracted to the Light.
But there are far too many Christians who are not unlike the Pharisees in the first century, so committed to a narrow way of thinking they are no longer benefiting their culture and they are more like a dim lamp underneath a basket!