Mark 9:2-10
2 After six days Jesus took Peter, James, and John and led them up a high mountain apart by themselves. And he was transfigured before them,
3 and his clothes became dazzling white, such as no fuller on earth could bleach them.
4 Then Elijah appeared to them along with Moses, and they were conversing with Jesus.
5 Then Peter said to Jesus in reply, "Rabbi, it is good that we are here! Let us make three tents: one for you, one for Moses, and one for Elijah."
6 He hardly knew what to say, they were so terrified.
7 Then a cloud came, casting a shadow over them; then from the cloud came a voice, "This is my beloved Son. Listen to him."
8 Suddenly, looking around, they no longer saw anyone but Jesus alone with them.
9 As they were coming down from the mountain, he charged them not to relate what they had seen to anyone, except when the Son of Man had risen from the dead.
10 So they kept the matter to themselves, questioning what rising from the dead meant
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I'll again defer to greater minds than mine. This is from the Catena Aurea, compiled by Aquinas in the 13th century. It's truly a great work, stepping through the gospels verse by verse and uniting commentary from the first millenium of the Church. In this passage he gives us commentary from Chrysostom (3rd - 4th century), Bede (7th - 8th centurty), and Origen (3rd century).
Chrysostom: He brings Moses and Elias before them; first, indeed, because the multitudes said that Christ was Elias, and one of the Prophets. He shews 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.”
Bede: If the transfigured humanity of Christ and the society of but two saints seen for a moment, could confer delight to such a degree that Peter would, even by serving them, stay their departure, how great a happiness will it be to enjoy the vision of Deity amidst choirs of Angels forever?
It goes on, “For he wist not what to say;” although, however, Peter from the stupor of human frailty knew not what to say, still he gives a proof of the feelings which were within him; for the cause of his not knowing what to say, was his forgetting that the kingdom was promised to the Saints by the Lord not in any earthly region, but in heaven; he did not remember that he and his fellow Apostles were still hemmed in by mortal flesh and could not bear the state of immortal life, to which his soul had already carried him away, because in our Father’s house in heaven, a house made with hands is not needed.But again even up to this time he is pointed at, as an ignorant man, who wishes to make three tabernacles for the Law, the Prophets, and the Gospel, since they in no way can be separated from each other.
Chrysostom: Again, Peter neither comprehended that the Lord worked His transfiguration for the shewing forth of His true glory, nor that He did this in order to teach men, nor that it was impossible for them to leave the multitude and dwell in the mountain.It goes on, “For they were sore afraid.”But this fear of theirs was one by which they were raised from their usual state of mind to one higher, and they recognised that those who appeared to them were Moses and Elias. The soul also was drawn on to a state of heavenly feeling, as though carried away from human sense by the heavenly vision.
Origen: Mark says in his own person, “For he wist not what to say.” Where it is matter for consideration, whether perchance Peter spoke this in the confusion of his mind, by the motion of a spirit not his own; whether perchance that spirit himself who wished, as far as in him lay, to be a stumbling block to Christ, so that He might shrink from that Passion, which was the saving of all men, did not here work as a seducer and wish under the colour of good to prevent Christ from condescending to men, from coming to them, and taking death upon Himself for their sakes.
Bede: Now because Peter sought for a material tabernacle, he was covered with the shadow of the cloud, that he might learn that in the resurrection they are to be protected not by the covering of houses, but by the glory of the Holy Ghost.Wherefore it goes on, “There was a cloud that overshadowed them.”And the reason why they obtained no answer from the Lord was that they asked unadvisedly; but the Father answered for the Son. Wherefore there follows, “And a voice came out of the cloud, saying, This is My beloved Son, in whom I am well pleased.”
Chrysostom: The voice proceeded from a cloud in which God is wont to appear, that they might believe that the voice was sent forth from God. But in that He says, “This is My beloved Son,” He declares that the will of the Father and the Son is one, and that, save in that He is the Son, He is in all things One with Him who begot Him.
Bede: He then whose preaching, as Moses foretold, every soul that wished to be saved should hear when He came in the flesh, He now come in the flesh is proclaimed by God the Father to the disciples as the one whom they were to hear.
