Salt of the earth
Many know that salt renders more vivid the flavor of the food; in addition, salt also stimulates our sense of taste. However, in Jesus' time, salt does even more. The common fuel for cooking at that time is not wood, which was scarce, but camel or donkey dung, which was plentiful. The dung has to mix with salt which has catalytic properties and thus cause the dung to burn. If we ponder this simple practice, we begin to see how important the role of Christians is in the world.
When Jesus tells the disciples that they are the salt of the earth, they are in truth called to communicate the true flavor of life in Christ to those around them; to stimulate a new taste for the Good News and the goodness of God's grace. But that is not enough, they are also called to be a catalyst, that is, to set the world on fire for Christ; to open up the capacity of others to receive Christ's words and to extend the love of Christ in the circumstances of the lives of others.
Light of the world
In the Old Testament, the prophet Isaiah has foretold a light will come upon the people and upon the land of darkness (rf Is 9:1). In John's Gospel, Jesus identifies Himself as “the light of the world” (Jn 9:5). Jesus indeed is the one who comes to bring the light of salvation into the darkness of sin and death.
When Jesus tells His disciples that they are the light of the world, they are in truth called to enlighten the world with the apostolic preaching, to illuminate mankind for conversion and to awaken them the source and purpose of life. The true light is Jesus Christ and the disciples are the light bearers. It is clear that the light of the disciples depends upon the presence of Jesus in their midst.
So as Christians, we are privileged to have these God-given dignities as salt and light for the world, but it is also our responsibility to uphold this dignity. Everything that a Christian is and does draws vitally from Christ, the source of taste and lumination. If salt loses its saltiness, it loses its essential function and thus becomes useless. Likewise, as God has revealed Himself for all to see in Jesus the Light, the light bearers are to share and preserve this light by their good teachings, deeds and examples. Hence Christians, by their very nature as believers in Christ, should “put on the Lord Jesus Christ”, that is, to clothe themselves with Christ to reveal the glory of God by responding to the call of being the salt and light to the world (rf Rm 13:14).