You are the salt of the earth, but if salt has lost its taste, how shall its saltiness be restored? It is no longer good for anything except to be thrown out and trampled under people’s feet.

You are the light of the world. A city set on a hill cannot be hidden. Nor do people light a lamp and put it under a basket, but on a 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 who is in heaven.

—Matthew 5:13-16

From John Stott’s article “Four Ways Christians Can Influence the World“:

In both these metaphors of the salt and the light, Jesus teaches about the responsibility of Christians in a non-Christian, or sub-Christian, or post-Christian society. He emphasizes the difference between Christians and non-Christians, between the church and the world, and he emphasizes the influences Christians ought to have on the non-Christian environment. The distinction between the two is clear.

The world, he says, is like rotting meat. But you are to be the world’s salt.

The world is like a dark night, but you are to be the world’s light.

This is the fundamental difference between the Christian and the non-Christian, the church and the world.

Then he goes on from the distinction to the influence.

Like salt in putrefying meat, Christians are to hinder social decay.

Like light in the prevailing darkness, Christians are to illumine society and show it a better way.

It’s very important to grasp these two stages in the teaching of Jesus. Most Christians accept that there is a distinction between the Christian and the non-Christian, between the church and the world. God’s new society, the church, is as different from the old society as salt from rotting meat and as light from darkness.

But there are too many people who stop there; too many people whose whole preoccupation is with survival—that is, maintaining the distinction.

The salt must retain its saltiness, they say. It must not become contaminated.

The light must retain its brightness. It must not be smothered by the darkness.

That is true. But that is merely survival. Salt and light are not just a bit different from their environment. They are to have a powerful influence on their environment.

The salt is to be rubbed into the meat in order to stop the rot.

The light is to shine into the darkness. It is to be set upon a lamp stand, and it is to give light to the environment.

That is an influence on the environment quite different from mere survival.