Mark 9:8

(ubi sup.) 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. There follows, And suddenly, when they had looked round about, they saw no man any more, save Jesus only with themselves; for as soon as the Son was proclaimed, at once the servants disappeared, lest the voice of the Father should seem to have been sent forth to them.

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.