IV. The significance of the appearance of Moses and Elias
For Moses and Elias, that is the Law and the Prophets, appeared talking with the Lord; that in the presence of those five men might most truly be fulfilled what was said: “In two or three witnesses stands every word.” What more stable, what more steadfast than this word, in the proclamation of which the trumpet of the Old and of the New Testament joins, and the documentary evidence of the ancient witnesses combine with the teaching of the Gospel? For the pages of both covenants corroborate each other, and He Whom under the veil of mysteries the types that went before had promised, is displayed clearly and conspicuously by the splendour of the present glory. Because, as says the blessed John, “the law was given through Moses: but grace and truth came through Jesus Christ,” in Whom is fulfilled both the promise of prophetic figures and the purpose of the legal ordinances: for He both teaches the truth of prophecy by His presence, and renders the commands possible through grace.
Source: Sermons (New Advent)