The Lord Jesus unfolded these two metaphors in His discourse on the kingdom in Matthew 5-7. In His teaching, He revealed the spiritual character and the characteristics of His kingdom and set forth its elevated nature to His disciples.
The Lord commenced this teaching with special reference to the spiritual favour that God has bestowed on His own; the beatitudes set forth these blessings in all of their bounty and fulness. It has been noted that in the structure of the nine beatitudes, the first four stress our relationship with God. The Lord Jesus describes the poor in spirit, the mournful, the meek, and those who hunger and thirst after righteousness. The emphasis is placed on our relationship with Him. The conclusion of the beatitudes focuses on our relationship with the world. We see its hatred and animosity toward those who follow Christ and are part of His kingdom. The Lord intimated, first to His own and then by extension to us, what we should expect from the world.
It is in this context, where the disciples’ relationship to the King, His kingdom and then the world are introduced, that these two aspects and metaphors are highlighted.
The Difference in the Metaphors
The two metaphors used by the Lord Jesus are evidently very different. Salt is that which preserves; light is that which is seen and which shines forth. It speaks in principle of that which makes God known. Salt acts in a private way and in a sphere which may not be noticed; light is set forth and seen in a more public setting. These speak of the two spheres of every believer’s life. We have responsibilities in our private lives and our public relationships to represent God before men.
The Details of the Metaphors
The Lord’s words are very clear: we are not like salt and light – we are salt and light! What is being stressed are the facts; the Lord is making a statement about His own and what they are in their character. Then Christ presents a contrast. Salt is an agent that is a preservative; it is the enemy of decay. It is contrasted with the earth which is corrupt. The fact that we are the light of the world suggests that the world is in darkness. We have been left in a world marked by moral decay and degenerate darkness. We are expected to have an arresting influence and an illuminating impact for our Lord.
The Development of the Metaphors
The development of these metaphors is seen in the Old Testament. The second time salt is mentioned in the Scriptures is in Genesis 19:26: Lot’s wife became a pillar of salt. This was the manifestation of the permanence of God’s judgement, salt showing forth and being linked to something permanent. This is also seen in the covenant of salt (2Ch 13:5). Such a covenant was made by grains of salt being transferred between two parties. Once accepted, these were mingled in the parties’ respective bags of salt. These particles couldn’t be retrieved, showing that a covenant, once made, could never be broken. This again emphasizes permanence. The aspect of permanence may not be the primary focus in Matthew’s context; however, the Lord does speak of salt losing its savour and thus its effectiveness. He expects us to remain unchanging in our character, standing against the evils of the world system that is always evolving and mutating.
The first time we have light mentioned in Scripture is in Genesis 1:3: “And God said, Let there be light.” Light is something which is linked to God Himself, who is the source of light. This is taken up by Paul, who reminds us, “For God, who commanded the light to shine out of darkness, hath shined in our hearts, to give the light of the knowledge of the glory of God in the face of Jesus Christ” (2Co 4:6). We are those who have been enlightened, and as such we are expected to show forth that light to others.
The Duties in the Metaphors
We should have an influence for good on those with whom we come into contact. The areas in which both the salt and light are expected to operate are clear; the Lord imposed no geographical restrictions. We are the salt of the earth and the light of the world; this is one of the reasons why we were not taken home to glory immediately upon our conversion. In this world, we represent the interests of our Lord. We have no mandate to become engaged with the world around us. The Lord has not asked us to entangle ourselves in social, environmental, political or religious causes, but we are expected to have a preserving influence in the sphere in which the Lord has placed us. We provide light where we live; the Lord indicated this by His reference to a city on a hill and also by His reference to the house – “it giveth light unto all that are in the house” (Mat 5:15). God expects that our homes will be used in a way that will show forth the praises of Him who hath called us out of darkness. Many of our neighbours will never come to a gospel meeting; God has placed us in our present location so that we will reveal and manifest Him to them.
The Dangers in the Metaphors
There are two dangers noted: it is possible for salt to lose its savour and for light to be placed under a bushel or basket, meaning it will not be seen. We know that salt is a stable compound. In what way, then, could it lose its savour or effectiveness? It has been noted that when salt was mined from areas like the Dead Sea, there would have been a lot of impurities and it would be mixed with other substances; this would mean it was less effective. We need to heed the Lord’s warning. If we engage in things that are not wholesome, they will bring mixture into our lives; the effectiveness of our testimony and our preserving influence will be lost. The same principle is seen in bearing light. It is possible to hide our light; it can be difficult to speak publicly to those with whom we live and work. God help us to rely on Him for strength so that we will be enabled to exhibit His character and show forth His light among men.
 Bible quotations in this article are from the KJV.