From Tim Keller’s convicting and humbling sermon, “Salt and Light” (March 11, 1990), as published in The Timothy Keller Sermon Archive (New York City: Redeemer Presbyterian Church, 2013):

When Jesus Christ says, “You are the salt and the light of the world,” this is what he is saying a Christian should be like. Okay, now hold your breath. Number one, salt and light expose decay and darkness. If you are light, that means your life should be so beautiful that when it comes into contact with other parts of the environment, the beauty of your life shows up other things for what they really are.

For example, if you’re a Christian, then just by your very presence, you show up, you reveal the dishonesty in the business. You reveal the gossip in the office. You reveal the racism in your neighborhood. You reveal the corruption in your political ward. You reveal the promiscuity in your party … just simply by being a Christian. You walk on in and it immediately makes the racism look like racism. It makes the promiscuity look like promiscuity. It makes the gossip look like gossip. It makes the corruption look like corruption, just by you saying, “I’m going to live according to the truth, which is the Ten Commandments, to the beauty of Jesus Christ.”

If your life, by its order, by the way in which you handle pressure, by the way in which you take criticism, by the way in which you treat the people who work under you, if you are like Jesus Christ, the beauty of that is going to show up the reality of the environment. A good light shows you real color, right?

Have you ever noticed that sometimes you pull out a pair of socks, and you can’t tell if they’re blue or black, and you look in one light and you still can’t tell, and you have to come to a good light in order to tell whether it’s blue or black? A real good light shows you the real colors. If you are a Christian walking like Jesus Christ, then the beauty of your life shows everybody around you what is good and what is bad.