It is worth considering that of the three prophets assumed by God in the Old Testament, two of them appeared with the glorified Christ on Mount Tabor: Moses, giver of the Law, and Elias, who slew false prophets on Mount Carmel. The message God entrusted to both of them was that God demanded due reverence in worship and deportment from His people. The message of Christ, more difficultly, was that God asked something proactive of mankind, to love God with all one's heart and one's neighbor as one's self.
Christ's Transfiguration displayed His albescent divinity, what St. Gregory Palamas called the "uncreated divine energies" (today that phraseology would be dangerously New Age). Perhaps we should recall that His divine also illuminated our humanity....
"For we have not by following artificial fables, made known to you the power, and presence of our Lord Jesus Christ; but we were eyewitnesses of his greatness. For he received from God the Father, honour and glory: this voice coming down to him from the excellent glory: This is my beloved Son, in whom I am well pleased; hear ye him." (2Peter 1:16-17, first nocturne of Mattins)
"Of this metamorphosis the chief work was to remove from the hearts of the disciples the stumbling at the Cross. Before their eyes was unveiled the splendour of His hidden majesty, that the lowliness of His freely-chosen suffering might not confound their faith. But none the less was there here laid by the Providence of God a solid foundation for the hope of the Holy Church, whereby the whole body of Christ should know with what a change it is yet to be honoured. The members of that body whose Head hath already been transfigured in light may promise themselves a share in His glory." (Pope St. Leo the Great, second nocturne of Mattins)