From certain perspectives, you could say the world is constantly improving. Science and technology have helped all but eradicate many diseases that once plagued the world. Information has never been more accessible, and keeping in touch with other people has never been easier.
But as John MacArthur explains, those advances can’t fully mitigate the world’s consistent downward spiral.
Man has increased in scientific, medical, historical, educational, psychological, and technological knowledge to an astounding degree. But he has not changed his own basic nature and he has not improved society. Man’s knowledge has greatly improved, but his morals have progressively degenerated. His confidence has increased, but his peace of mind has diminished. His accomplishments have increased, but his sense of purpose and meaning have all but disappeared. Instead of improving the moral and spiritual quality of his life, man’s discoveries and accomplishments have simply provided ways for him to express and promote his depravity faster and more destructively. Modern man has simply invented more ways to corrupt and destroy himself.  John MacArthur, The MacArthur New Testament Commentary: Matthew 1-7 (Chicago: Moody Press, 1985), 238.
In short, he says, “The biblical world view is that the world is corrupted and decayed, that it is dark and darkening.” The MacArthur New Testament Commentary: Matthew 1-7, 237. Apart from that perspective, Christ’s words in Matthew 5:13-16 make no sense.
You are the salt of the earth; but if the salt has become tasteless, how can I be made salty again? It is no longer good for anything, except to be thrown out and trampled underfoot by men. You are the light of the world. A city set on a hill cannot be hidden; nor does anyone light a lamp and put it under a basket, but on the lampstand, and it gives light to all who are in the house. Let your light shine before men in such a way that they may see your good works, and glorify your father who is in heaven.
The pervasive decay and darkness of the world are unmistakable. In this short passage from the Sermon on the Mount, Christ is calling His followers—and in particular, His disciples—to stand apart from the corruption that consumes the world. More than that—we’re called to exert a sanctifying influence in this sinful society.
We cannot reflect the world’s corruption back to it. Nor can we simply withdraw from society altogether. Instead, as John MacArthur writes, Christ has called us to be a preserving and illuminating force in the world.
The church cannot accept the world’s self-centeredness, easy solutions, immorality, amorality, and materialism. We are called to minister to the world while being separated from its standards and ways. Sadly, however, the church today is more influenced by the world than the world is influenced by the church.
In both verse 13 and verse 14 the pronoun you is emphatic. The idea is, “You are the only salt of the earth” and “You are the only light of the world.” The world’s corruption will not be retarded and its darkness will not be illumined unless God’s people are its salt and light. The very ones who are despised by the world and persecuted by the world are the world’s only hope. The MacArthur New Testament Commentary: Matthew 1-7, 239-40.
In simple terms, we are to be in the world, but not of it.
In this short passage, Christ used two practical analogies to make His point clear to His audience. John explains the significance of the Lord’s first illustration: salt.
Salt has always been valuable in human society, often much more so than it is today. During a period of ancient Greek history it was called theon, which means divine. The Romans held that, except for the sun, nothing was more valuable than salt. Often Roman soldiers were paid in salt, and it was from that practice that the expression “not worth his salt” originated. . . .
In numerous ways Jesus’ hearers-whether Greek, Roman, or Jewish-would have understood “salt of the earth” to represent a valuable commodity. Though most could not have understood His full meaning, they knew He was saying that His followers were to have an extremely important function in the world. Whatever else it may have represented, salt always stood for that which was of high value and importance. The MacArthur New Testament Commentary: Matthew 1-7, 241.
Several scholars and commentators have suggested a variety of interpretations regarding the specific quality of salt Christ was referring to. Some say the Lord was talking about the purity of salt and the need for Christians to be similarly pure in heart. Others claim Christ was teaching that Christians simply add flavor to the world. Some believe the point was that Christians are to bring a sting to the world, making it uncomfortable. Still others argue that, like salt, Christians are supposed to create a thirst in the world for God and His truth.
While all those interpretations have some measure of validity, John MacArthur makes a convincing case that Christ had a different characteristic in mind when He said His followers were the salt of the earth.
I believe the primary characteristic Jesus emphasizes is that of preservation. Christians are a preserving influence in the world; they retard moral and spiritual spoilage. When the church is taken out of the world at the rapture, Satan’s perverse and wicked power will be unleashed in an unprecedented way (see 2 Thessalonians 2:7–12). Evil will go wild and demons will be almost unbridled. Once God’s people are removed it will take only seven years for the world to descend to the very pits of hellishness (see Daniel 9:27; Revelation 6–19). The MacArthur New Testament Commentary: Matthew 1-7, 243.
Serving as the salt of the earth—as God’s own agents of preservation against the creeping decay of this dying world—is a tremendous responsibility. One that we must take seriously, and guard against forfeiting.
There is a sense in which salt cannot really become unsalty. But contamination can cause it to lose its value as salt. Its saltiness can no longer function.
Jesus is not speaking of losing salvation. God does not allow any of His own to be taken from Him. “My sheep hear My voice, and I know them, and they follow Me; and I give eternal life to them, and they shall never perish; and no one shall snatch them out of My hand,” Jesus assures us (John 10:27). Christians cannot lose their salvation, just as salt cannot lose its inherent saltiness. But Christians can lose their value and effectiveness in the kingdom when sin and worldliness contaminate their lives, just as salt can become tasteless when contaminated by other minerals. It is a common New Testament truth that although true believers are identified as righteous, godly, and salty, there are times when they fail to be what they are (cf. Romans 7:15–25), which Peter says leads to loss of assurance (2 Peter 1:9–10), not loss of salvation.
With great responsibility there is often great danger. We cannot be an influence for purity in the world if we have compromised our own purity. We cannot sting the world’s conscience if we continually go against our own. We cannot stimulate thirst for righteousness if we have lost our own. We cannot be used of God to retard the corruption of sin in the world if our own lives become corrupted by sin. To lose our saltiness is not to lose our salvation, but it is to lose our effectiveness and to become disqualified for service (see 1 Corinthians 9:27). The MacArthur New Testament Commentary: Matthew 1-7, 245-46.
Ultimately, if we’re going to have any preserving influence in the world at all, we must first be distinct from it. We must be set apart, and maintain that distance if we have any hope of influencing the world for the sake of God’s kingdom.
But it’s not just about keeping our distance. As we’ll see next time, God has not just called us to preserve the world, but to illuminate it with His truth.