You are the salt of the earth. But if the salt loses its saltiness, how can it be made salty again? It is no longer good for anything, except to be thrown out and trampled underfoot. You are the light of the world. A city on a hill cannot be hidden. Neither do people light a lamp and put it under a bowl. Instead they put it on its stand, and it gives light to everyone in the house.
In a previous post I noted that just as the function of salt is to preserve and enhance food, so too Jesus’s disciples must preserve and enhance what good God has given to the world. Similarly, Jesus says “You the light of the world.” A light is of no use unless it illuminates. When camping I use a headlamp to light up my way in the wilderness. Imagine being in the dark of the woods, turning on the headlamp and, instead of putting it on my head, I put it into my backpack! For the light to be useful it must illumine what is around it. Of course, as a follower of Jesus the source of my light is Jesus (cf. John 8:12. See also, Eph 5:8; Philip 2:15). Apart from Christ, my light is merely a reflection of my own efforts, which are far from radiant!
On what does the light shine? Matthew 5:16 is explicit: “In the same way, let your light shine before others, that they may see your good deeds and glorify your Father in heaven.” Whatever good I do because of Christ in me must be visible to the world around me! It is through me and by Christ that God receives glory.
Just as the “you” was emphatic in the first metaphor, “You are the salt of the earth,” so too is it emphatic here, “You are the light of the world.” This passage of Scripture tells me that, though I am to be distinguishable from the world (just as salt is distinguishable from the food that it preserves or light is distinguishable from darkness), I must engage the world in positive ways that point the way to God. I cannot retreat from the world; I must not isolate my faith. Being useful means bringing something unique to the world that it does not already have and is in need of. Being useful means being an agent of change that impedes moral, economic, political, social, cultural, relational, and intellectual decay. Being useful means shining the light of good behavior so those in the dark will take note and look up toward God as the source of that goodness.