After the Beatitudes, in the Sermon on the Mount (Matthew 5-7) Jesus offers a series of comments on a variety of topics, beginning with his view of who and what his disciples are and are supposed to be. In brief, they are “the salt of the earth” and “the light of the world” whose light shines and deeds inspire.
Salt was of inestimable value in a climate in which intense heat spoiled food rapidly. Its grains were valued throughout the Greco-Roman world so much that Roman soldiers were paid in packets of salt, their salarium, from which we get the word “salary.”
Similarly, in the typical first-century Palestinian village, the one-room home in which most people lived had a single lamp. This provided the only household light, even during the day, since windows were small and usually kept covered to keep out the sun and its searing heat.
In verse 5:16, the salt and light are the “good deeds” of the disciples, to be given as signs so that others can see and glorify “your Father who is in heaven.”
Compare this to Christian people and institutions through the modern age.
The false and bland message of Christian respectability, social manners and adherence to civil laws – even when they are unjust – is the sort of salt that long ago lost its savor. The teachings of guilt, prejudice and conflict cast a pall on the gospel’s light, leaving humanity in greater fear, confusion and suffering than before.
No one can savor the salt and see the light, thus many (most?) people disbelieve there even is a Father in heaven. How could there be? Why believe when even the believers have no salt to offer and no light to show the way?
According to the gospel, evangelism is something quite different from marketing campaigns to fill churches and collection baskets for buildings and clerics, incense and chalices, false promises and prophets.
Jesus only speaks of providing an example of faith in action, in other words, good deeds. Do what is good, then shut up; people will get the message.