14 You are the light of the world. . . .
Gloss: As the doctors by their good conversation are the salt with which the people is salted; so by their word of doctrine they are the light by which the ignorant are enlightened.
Pseudo-Chrysostom: 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.
Augustine: By the world here we must not understand heaven and earth, but the men who are in the world; or those who love the world for whose enlightenment the Apostles were sent.
Hilary: It is the nature of a light to emit its rays whithersoever it is carried about, and when brought into a house to dispel the darkness of that house. Thus the world, placed beyond the pale of the knowledge of God, was held in the darkness of ignorance, till the light of knowledge was brought to it by the Apostles, and thenceforward the knowledge of God shone bright.
Remigius: As the sun sends forth his beams, so the Lord, the Sun of righteousness, sent forth his Apostles to dispel the night of the human race.
Chrysostom: Mark how great His promise to them, men who were scarce known in their own country that the fame of them should reach to the ends of the earth. The persecutions which He had foretold, were not able to dim their light, yea they made it but more conspicuous.
14 . . . A city seated on a mountain cannot be hid.
Chrysostom: Thus shewing them that they ought to be careful of their own walk and conversation, seeing they were set in the eyes of all, like a city on a hill, or a lamp on a stand.
Pseudo-Chrysostom: This city is the Church of which it is said, “Glorious things are spoken of thee, thou city of God” [Ps 87:3]. Its citizens are all the faithful, of whom the Apostle speaks, “Ye are fellow-citizens of the saints” [Eph 2:19]. It is built upon Christ the hill, of whom Daniel thus, “A stone hewed without hands” [Dan 2:34] became a great mountain.
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. I (London: Rivington, 1842).