Jesus’ disciples are salt and light. The salt is low, on the earth; the light is set up high on a lamp, so it can light the house. James Jordan has pointed out the connection with earth (salt) and heaven (light), and this links also to the Abrahamic promise of an abundant seed that is like sand and stars. Jesus’ disciples, like Israel, are to be an exalted heavenly people, the people that makes the world – like the salted offerings – a pleasant aroma to Yahweh.
But if the salt loses savor, it is trampled underfoot. The only other time this image occurs in Matthew is 7:6, where it refers to swine trampling pearls underfoot. Stassen makes a good case that 7:6 refers to Romans, and that Jesus is warning about the threat that Israel will be trampled by the Gentiles (cf. Luke 21:24).
This sheds some light on 5:13: Israel had ceased to be faithful to the covenant, ceased to be salty, and were in danger of being trampled under the feet of Gentiles. Jesus’ disciples should take warning from Israel: If they lose savor, they will likewise be trampled. They will not be lights lifted up, but low, mingled with the dust.