Remember salt pork? It was not the most wonderful thing to eat, but it made quite a difference. There are those people when eating a meal, even before they taste the meal it must be covered with salt. There are foods that must be eaten with salt, at least in the opinion of some people. French fries. Potato chips. Eggs. Salt adds flavor. But salt also protects. Salt helps to preserve foods from going bad. In an age without refrigeration, salt was an important ingredient in any kitchen.
By using this image, Jesus helps to create the analogy to faith, which has similar qualities. Faith, too, adds flavor to life. Faith too, preserves and protects important things. A disciple is a witness. A disciple has discovered how it is that faith protects. And a disciple is one who makes sure to preserve the openness to God, to his grace, so that the flavor that changes life is not lost.