2 The Lord Jesus Himself shone bright as the sun; His raiment became white as the snow; and Moses and Elias talked with Him. Jesus Himself indeed shone as the sun, signifying that “He is the light which lights every man that comes into the world.” What this sun is to the eyes of the flesh, that is He to the eyes of the heart; and what that is to the flesh of men, that is He to their hearts. Now His raiment is His Church. For if the raiment be not held together by him who puts it on, it will fall off.
Of this raiment, Paul was as it were a sort of last border. For he says himself, “I am the least of the Apostles.” And in another place, “I am the last of the Apostles.” Now in a garment the border is the last and least part. Wherefore as that woman which suffered from an issue of blood, when she had touched the Lord's border was made whole, so the Church which came from out of the Gentiles, was made whole by the preaching of Paul. What wonder if the Church is signified by white raiment, when you hear the Prophet Isaiah saying, “Though your sins be as scarlet, I will make them white as snow”? Moses and Elias, that is, the Law and the Prophets, what avail they, except they converse with the Lord?
Except they give witness to the Lord, who would read the Law or the Prophets? Mark how briefly the Apostle expresses this; “For by the Law is the knowledge of sin; but now the righteousness of God without the Law is manifested:” behold the sun; “being witnessed by the Law and the Prophets,” behold the shining of the Sun.
Source: Sermons on Selected Lessons of the New Testament (New Advent)