Mark 9:4

(Hom. in Matt. 56) He brings Moses and Elias before them; first, indeed, because the multitudes said that Christ was Elias, and one of the Prophets, He shows Himself to the Apostles with them, that they might see the difference between the Lord, and His servants. And again because the Jews accused Christ of transgressing the law, and thought Him a blasphemer, as if He arrogated to Himself the glory of His Father, He brought before them those who shone conspicuous in both ways; for Moses gave the Law, and Elias was zealous for the glory of God; for which reason neither would have stood near Him, if He had been opposed to God and to His law. And that they might know that He holds the power of life and of death, He brings before them both Moses who was dead, and Elias who had not yet suffered death. Furthermore He signified by this that the doctrine of the Prophets was the schoolmaster to the doctrine of Christ. He also signified the junction of the New and Old Testament, and that the Apostles shall be joined in the resurrection with the Prophets, and both together shall go forth to meet their common King. It goes on, And Peter answered and said to Jesus, Master, it is good for us to be here; and let us make three tabernacles, one for thee, and one for Moses, and one for Elias.

(ubi. sup.) Moses and Elias, of whom one, as we read, died, the other was carried away to heaven, signify the coming glory of all the Saints, that is, of all who in the judgment-time are either to be found alive in the flesh, or to be raised up from that death of which they tasted, and who are all equally to reign with Him.

Or else it means, that we are to see in glory both the Law and the Prophets speaking with Him, that is, we shall then find that all those things which were spoken of Him by Moses and the other prophets agree with the reality; then too we shall hear the voice of the Father, revealing to us the Son of the Father, and saying, This is my beloved Son, and the cloud, that is, the Holy Ghost, the fount of truth, will overshadow us.