"You are the salt of the earth; but if salt has lost its taste, how can its saltiness be restored? It is no longer good for anything, but is thrown out and trampled underfoot. You are the light of the world. A city built on a hill cannot be hid. No one after lighting a lamp puts it under the bushel basket, but on the lamp stand, and it gives light to all in the house. In the same way, let your light shine before others, so that they may see your good works and give glory to your Father in heaven." (Matthew 5:13-14) (NRSV)
Jesus moves now to the role his followers are to play in human society: salt and light.
Most Americans tend to think of salt as a seasoning. In the ancient world, salt was used primarily as a preservative. Those listening to Jesus speak would have heard him saying, "You are the preservative of the world." Translations generally fail to capture Jesus' intent when he speaks of salt losing its flavor. In the Greek, it's clear Jesus had contamination in mind. A good paraphrase would be: "But if salt has become adulterated with other materials, it loses it's ability to preserve."
Verse 14 introduces the metaphor of light. Jesus provides two images: a city set on hill, and a lamp lit inside a dwelling. In order to appreciate the metaphor, most of us have to imagine a world in which night time was really dark. Street lights, office lights, traffic lights, light spilling through the windows of homes, light reflected from our great cities--it's hard to find genuine darkness these days. Ask amateur astronomers, if you doubt me!
The first century world, though, knew darkness. When the sun went down, and clouds covered the sky, and you were in the wilderness, pitch darkness engulfed you. Darkness was frightening precisely because it was thick, you could not see, and anything might be "out there." A traveler in the wild would have been grateful even for the dim, reflected light of a distant city. No one would have placed a bushel basket over a lit lamp, but instead would have hastened to place it on a lampstand and so bring light to a small room.
Light and salt were vital to life in the first century. Jesus now places his followers in a similar category: well-lived, their words and deeds and attitudes will prove crucial to others and to human society.
The preceding beatitudes provide the heart of what it means to live life well. Later in the sermon, Jesus will add depth and breadth to his teachings, but the core of the matter is the beatitudes.
The Christian life, or the life of anyone who follows Jesus, cannot be solely private. In essence, Jesus commissions his followers to plunge into but not lose themselves in the life of society. Their mission is to preserve the way of God among humans by walking such a path themselves, to illumine the way of God by their lives.
Salt works quietly and unseen, light lessens or banishes darkness without fanfare. Jesus is not commissioning an army, creating a school of theology, tying his hopes to politics, or fashioning an institution. Instead, he is trying to mold the hearts and minds of individuals, set them loose in culture, and inspire them to live true to his intentions. Jesus appears content to hope in the efficacy of such an approach.