“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 they may see your good works and give glory to your Father who is in heaven.†~Matthew 5:13-16
I often hear it said that we, as Christians, are commanded to be salt and light. However, that statement is not entirely accurate.
In this passage, Jesus is not giving a command. He is stating a fact. Believers – simply because they belong to Christ – are salt and light.
Salt
Before modern modes of refrigeration and preservation, salt was used as a preservative. It would slow the process of decay when rubbed into a piece of meat.
In the same way, Christians are a preserving influence on the decaying world around us. When functioning as they should, they slow the moral and spiritual corruption going on all around us.
“…but if the salt has lost its taste, how shall its saltiness be restored?â€
In the strictest, technical, scientific sense, salt cannot lose its saltiness. But, much of the salt used in the time that this passage was written was harvested from the shores of theDead Sea, and often contained many impurities or contaminants. These contaminants often leached out the true salt. This left a substance that was virtually flavorless, or had a flavor that was absolutely repulsive. It could no longer be used for its intended purpose – as salt. It was worthless.
As believers, we cannot be a purifying influence on this world if our own purity has been compromised, or contaminated by the world. This is not to say that we can lose our salvation – we cannot. But, in becoming too much like the world we can lose our effectiveness for Christ.
“I know your works: you are neither cold nor hot. Would that you were either cold or hot! So, because you are lukewarm, and neither hot nor cold, I will spit you out of my mouth.†~Revelation 3:15-16
Instead of being the enhancing, preserving influence that we were designed to be, we render ourselves useless, lukewarm, and repulsive.
Light
The purpose of light is to illuminate darkness, and to expose that which is hidden by the darkness.
“Do all things without grumbling or questioning, that you may be blameless and innocent, children of God without blemish in the midst of a crooked and twisted generation, among whom you shine as lights in the world.†(Philippians 2:14-15)
Yes, in this passage, there is a command. But, that command is to “do all things without grumbling or questioning.â€Â Why? Because that is how we allow our light to shine. It is through our behavior, our actions, and our attitudes that the light of Christ shines in “the mist of a crooked and twisted generation.â€
“Therefore do not become partners with them; for at one time you were darkness, but now you are light in the Lord. Walk as children of light (for the fruit of light is found in all that is good and right and true), and try to discern what is pleasing to the Lord.  Take no part in the unfruitful works of darkness, but instead expose them. For it is shameful even to speak of the things that they do in secret. But when anything is exposed by the light, it becomes visible, for anything that becomes visible is light. Therefore it says, ‘Awake, O sleeper, and arise from the dead, and Christ will shine on you.’†~Ephesians 5:7-14
Our purpose, as believers, is to illuminate the dark world around us, and expose the works of wickedness, perverseness, and corruption around us. The command is not to be a light. As a Christian, you already are one.
The question is, then, does that light show? Is it effective? Is it serving its intended purpose?
“A city set on a hill cannot be hidden. Nor do people light a lamp and put it under a basket…†A light that is hidden, does not cease to be what it is: a light. It may be useless. It may not be serving the very purpose for which it was created. But, the fact remains, it is still a light.
“By definition, an influence must be different from that which it influences, and Christians therefore must be different from the world they are called to influence. We cannot influence the world for God when we are worldly ourselves. We cannot give light to the world if we revert to places and ways of darkness ourselves.†(The MacArthur New Testament Commentary (Matthew 1-7), pgs. 240-241)