In the Sermon on the Mount Jesus addresses his disciples and tells them that they are the salt of the earth and the light of the world.
"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 the same way, let your light shine before others, that they may see your good deeds and glorify your Father in heaven. (Mat 5:13-16)
Jesus uses the metaphors of salt and light because they share some common characteristics.
Salt and light both have qualities which are proactive in preventing things from going bad.
Just as salt is rubbed onto meat to prevent it from going bad so also the light prevents darkness from taking control. Salt has a cutting edge about it and light has an exposing element in it. Both expose corruption and stem the rot.
In those days it was a clay lamp which lit the whole house. It burned with olive oil drawn by a wick.
Jesus also said "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 a stand and it gives light to everyone in the house."
Friends, amid the tribulations of today, Jesus challenges us to be the 'light of the world.'