The world is evidently a dark place, with little or no light of its own, since an external source of light is needed it illumine it. True, it is “always talking about its enlightenment,” but much of its boasted light is in reality darkness. The world also manifests a constant tendency to deteriorate. The notion is not that the world is tasteless and that Christians can make it less insipid (“The thought of making the world palatable to God is quite impossible”), but that it is putrefying. It cannot stop itself from going bad. Only salt introduced from outside can do this. The church, on the other hand, is set in the world with a double role, as salt to arrest — or at least to hinder — the process of social decay, and as light to dispel the darkness.
When we look at the two metaphors more closely, we see that they are deliberately phrased in order to be parallel to each other. In each case Jesus first makes as affirmation (“You are the salt of the earth,” “You are the light of the world”). Then he adds a rider, the condition on which the affirmation depends (the salt must retain it saltiness, the light must be allowed to shine). Salt is good for nothing if its saltiness is lost; light is good for nothing if it is concealed.