But to live well must go before to teach well; hence after He had called the Apostles “the salt,” He goes on to call them “the light of the world.”
Or, for that salt preserves a thing in its present state that it should not change for the worse, but that light brings it into a better state by enlightening it; therefore the Apostles were first called salt with respect to the Jews and that Christian body which had the knowledge of God, and which they keep in that knowledge; and now light with respect to the Gentiles whom they bring to the light of that knowledge.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).