Theophylact of Ohrid — as excerpted in the Catena Aurea on Mark 9:1-8

“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.”