The Object and Meaning of the Transfiguration
And in this Transfiguration the foremost object was to remove the offense of the cross from the disciple's heart, and to prevent their faith being disturbed by the humiliation of His voluntary Passion by revealing to them the excellence of His hidden dignity. But with no less foresight, the foundation was laid of the Holy Church's hope, that the whole body of Christ might realize the character of the change which it would have to receive, and that the members might promise themselves a share in that honour which had already shone forth in their Head. About which the Lord had Himself said, when He spoke of the majesty of His coming, "Then shall the righteous shine as the sun in their Father's Kingdom," (Matthew 13:43) while the blessed Apostle Paul bears witness to the self-same thing, and says: "for I reckon that the sufferings of this time are not worthy to be compared with the future glory which shall be revealed in us" (Romans 8:18): and again, "for you are dead, and your life is hid with Christ in God. For when Christ our life shall appear, then shall you also appear with Him in glory." (Colossians 3:3) But to confirm the Apostles and assist them to all knowledge, still further instruction was conveyed by that miracle.
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." (Deuteronomy 19:15) 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," (John 1:17) 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.
-- Excerpt from Sermon 51 of St. Leo the Great
Detail from 15th Century Novgorod Icon of the Transfiguration