Theophylact of Ohrid
Catena Aurea · Markusevangelium 9
And he said unto them, Verily I say unto you, That there be some of them that stand here, which shall not taste of death, till they have seen the kingdom of God come with power. And after six days Jesus taketh with him Peter, and James, and John, and leadeth them up into an high mountain apart by themselves: and he was transfigured before them. And his raiment became shining, exceeding white as snow; so as no fuller on earth can white them. And there appeared unto them Elias with Moses: and they were talking with Jesus. 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. For he wist not what to say; for they were sore afraid. And there was a cloud that overshadowed them; and a voice came out of the cloud, saying, This is my beloved Son: hear him. And suddenly, when they had looked round about, they saw no man any more, save Jesus only with themselves.
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.