The modern trend with regards to the Transfiguration of our Redeemer is to emphasize His glory and divinity, which he revealed to Elias, Moses, Peter, James, and John. The Hesychast theology following the tradition of Palamas even extends this to include the manifestation of the uncreated, uncaused Divine energy of God. The ancient view is both simpler and more intuitive than either of these when we read the Gospel as a whole. From the Fathers in today's Mattins:
"The Lord takes chosen witnesses, and in their presence, reveals His glory. That form of body which He had in common with other men, He so transfigured with light, that His Face did shine as the sun, and His raiment became exceeding white as snow. 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." Sermon of St. Leo the Great on the Transfiguration, taken from the second nocturne
"Since the Lord had spoken much concerning dangers, much concerning His Own sufferings, much concerning death, and the killing of His disciples, and had laid upon them many hard and grievous things, and since all these were in this present life, and already hanging over them, whereas the good things were matter for hope and waiting as, for example, that whosoever should lose his life for His sake should find it, for that the Son of Man should come in the glory of His Father, and reward every man according to his works. Therefore, to assure them by their own eyes, and show them what the glory is wherein He will come, He manifested and unveiled it to them, as far as in this life they were able to grasp it, lest they and especially Peter should grieve over their own deaths, or the death of their Lord." 57th Sermon on Matthew by St. John Chrysostom, taken from the third nocturne
As an addendum, I would like to point out one of my favorite features of the icon of the Transfiguration. something that has always amused me. The three Apostles are blinded by the light and unable to see. John and James are looking away, overpowered by the radiance of the Lord's illuminated majesty. Peter, however, has to say something. Indeed, Peter always has to say something. Throughout the four Gospels he relentlessly speaks up at inopportune times and gets himself in trouble. After receiving the keys to the Kingdom of Heaven he passes on his chance to be quiet and earns the Lord's sharp condemnation "Get behind Me, Satan." There may be some theological significance to Peter's orientation toward the Lord in the icon—he did suggest the erection of three tabernacles—yet I cannot help but think there was an element of humor on the iconographers' part in having Peter attempt to get the Lord's attention while more important things transpire.