Jesus' audience at least partly includes "disciples" (5:1-2). Having described the appropriate lifestyle of disciples, Jesus now explains that a professed disciple who does not live this lifestyle of the kingdom is worth about as much as tasteless salt or invisible light-nothing.
Until my conversion in 1975 I professed to be an atheist in part because I looked at the roughly 85 percent of my fellow U.S. citizens who claimed to be Christians and could not see that their faith genuinely affected their lives. I reasoned that if even Christians did not believe in Jesus' teachings, why should I? My excuse for unbelief-and the excuse of many other secularists I knew-continued until God's Spirit confronted me with the reality that the truth of Christ does not rise or fall on the claims of his professed followers, but on Jesus himself. The faith of nominal Christians may appeal to non-Christians who can use it to justify their own unbelief, but such "Christians" will have no part in God's kingdom. Instead they will be thrown out and trampled (5:13).
Jesus refers here to more than good deeds; he refers to a good character (compare 7:17-20; 12:33-37). Such character comes only by embracing God's kingship as a gift (as in 10:40; 18:4, 12-14, 27). The images of salt and light evoke consideration less of what we do than of what we are. If only true disciples count before God (5:13-16) and true discipleship means treating both friends and enemies kindly (5:3-12), the salt-and-light paragraph becomes a resounding warning to heed Jesus' teaching on meekness in the preceding paragraph.
A disciple who rejects the beatitudes' values is like tasteless salt: worthless. Salt had a variety of uses (see Davies and Allison 1988:472-73); probably the most evident use was as a flavoring agent (Plut. Isis 5, Mor. 352F; Table-Talk 4.4.3, Mor. 669B). In any case the point is, what is to be done with salt that no longer functions as salt should?
A later Jewish story may illustrate how first-century hearers would have grasped Jesus' point. An inquirer reportedly asked a late first-century rabbi what to salt tasteless salt with; he responded, "The afterbirth of a mule" (b. Bekarot 8b). In that society everyone knew that mules are sterile; the point is, "You ask a stupid question, you get a stupid answer. Salt can't stop being salt!" But of course if it were to do so, it would no longer be of any value as salt.
Just as tasteless salt lacks value to the person who uses it, so does a professed disciple without genuine commitment prove valueless for the work of the kingdom.
A disciple whose life reveals none of the Father's works is like invisible light for vision: useless. Jesus reinforces his point with various images. A disciple should be as obvious as a city set on a hill (as most cities were), and a light in a home should be no easier to hide than a torchlit city at night (5:14-15; most homes had only one room). As a popular sage had put it, "What is the value of concealed wisdom, any more than of treasure that is invisible?" (Sirach 41:14).
Jesus depicts his disciples' mission in stark biblical terms for the mission of Israel. God called his people to be lights to the nations (for example, Is 42:6; 49:6)-that is, the whole world (compare Mt 18:7). Christians are light because-contrary to some psychoanalytic theories-their destiny (13:43) more than their past must define them.
But Christians cannot be content to remain the world's light in a merely theoretical sense; they must "be what they are," letting their light shine for their Father's honor (5:16). Ministers of the Word must equip all other Christians for their ministry as lights in their various neighborhoods and occupations (Eph 4:11-13; Tit 2:1, 5, 8, 10). While Jesus is opposed to our doing good works publicly for our own honor (6:1, "to be seen" by people), he exhorts us to do those good works publicly for God's honor (5:16; cf. 6:9). This distinction exhorts us to guard the motives of our hearts and consider the effects our public activities and pronouncements have on the spread of the gospel and the honoring of God among all groups of people.