Pseudo-Chrysostom

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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.