Mark 9:7

(Hom. in Matt. 56) 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.

(ubi sup.) 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.

(ubi sup.) And we must observe, that, as when the Lord was baptized in Jordan, so on the mountain, covered with brightness, the whole mystery of the Holy Trinity is declared, because we shall see in the resurrection that glory of the Trinity which we believers confess in baptism, and shall praise it all together. Nor is it without reason that the Holy Ghost appeared here in a bright cloud, there in the form of a dove; because he who now with a simple heart keeps the faith which he hath embraced, shall then contemplate what he had believed with the brightness of open vision. But when the voice had been heard over the Son, He was found Himself alone, because when He shall have manifested Himself to His elect, God shall be all in all, yea Christ with His own, as the Head with the body, shall shine through all things. (1 Cor. 15:28).

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.