The Lord Jesus Himself shone bright as the sun; His garment became white as the snow; and Moses and Elijah talked with Him. Jesus Himself indeed shone as the sun, signifying that He is “the true light that enlightens every man come into the world.” What the sun is to the eyes of the flesh, so He is to the eyes of the heart; and what that is to the flesh of men, that He is to their hearts…
Peter sees this, and as a man savoring the things of men says, “Lord, it is good for us to be here.” He had been wearied with the multitude. He had now found the mountain’s solitude; there he had Christ the Bread of the soul. What — should he depart once again to labor and suffering now that he had a holy love for God and a holy way of life? He wished well for himself; and so he added, “If you wish, I will make three booths here, one for you and one for Moses and one for Elijah.” To this the Lord made no answer; nevertheless, Peter received an answer. “He was still speaking, when lo, a bright cloud overshadowed them.” He wanted three tabernacles; the heavenly answer showed him that we have One, which human judgment desired to divide. Christ, the Word of God, the Word of God in the Law, the Word in the Prophets. Why, Peter, do you seek to divide them? Is it not more fitting for you to join them. You seek three; understand that they are but One.
As the cloud overshadowed them, and in a way made one tabernacle for them, “a voice from the cloud said, ‘This is my beloved Son.’” Moses was there; Elijah was there; yet it was not said, “These are My beloved sons.” For the Only Son is one thing; adopted sons another. He was singled out in whom the Law and the prophets glorified. “This is My beloved Son, in whom I am well pleased; hear Him!” Because you have heard Him in the Prophets, and you have heard Him in the Law. And where have you not heard Him? “When they heard this, they fell” to the earth. See then in the Church is exhibited to us the Kingdom of God. Here is the Lord, here the Law and the Prophets; but the Lord as the Lord. The Law in Moses, Prophecy in Elias — but they are servants and ministers. They are vessels: He is the fountain. Moses and the Prophets spoke and wrote; but when they poured out, they were filled from Him.
But the Lord stretched out His hand and raised them as they lay. And then “they saw no one but Jesus only.” What does this mean? When the Apostle was read, you heard, “For now we see through a glass darkly, but then face to face.” And “tongues shall cease,” when that which we now hope for and believe shall come. When they fell to the earth, they signified that we die, for it was said to the flesh, “You are dust, and to dust you shall return.” But when the Lord raised them up, He signified the resurrection. After the resurrection, what is the Law to you? what is Prophecy? Therefore neither Moses nor Elias is seen. Only He remains for you, He who “in the beginning was the Word, and the Word was with God, and the Word was God.” He remains for you, “that God may be all in all.” Moses will be there; but now no more the Law. We shall see Elijah there, too; but now no more the Prophet. For the Law and the Prophets have only given witness to Christ, that it befit Him to suffer, and to rise again from the dead on the third day, and to enter into His glory. (Source)