"Our Lord had spoken to His disciples many times not only concerning His Passion, Cross, and Death, but also concerning the coming persecutions and afflictions that they themselves would endure. Since all these evils were near at hand, but the enjoyment of good things which they hoped to receive in their stead was yet to come, our Savior desired to give them full assurance, evidently and openly, concerning that glory which is prepared for those who endure to the end. Therefore, fulfilling that which He had promised shortly before, that "there be some standing here which shall not taste of death, till they see the Son of man coming in His Kingdom" (Matt. 16:28), He took His three foremost disciples and ascended Mount Tabor, where He was transfigured before them. His face shone like the sun, and His clothes became white as the light. Suddenly, together with this dread and marvelous effulgence of light, there appeared those pinnacles of the Prophets, Moses and Elias, who spoke with the Lord Jesus concerning His saving Passion which was about to take place. Standing before Him as reverent servants, they showed that He is the Lord of both the living and the dead, for Moses came forth from Hades, having died many centuries before, and Elias, as it were from heaven, whither he had been taken up while yet alive. After a little while a radiant cloud overshadowed them and out of the cloud they heard that same voice which had been heard at the Jordan at the Baptism of Christ, testifying to the Divinity of Jesus and saying: "This is My beloved Son, in Whom I am well-pleased; hear ye Him" (Matt. 17: 5).
Such are the marvels, truly worthy of God, celebrated in this present feast, which is an image and prefiguring of the future state of the righteous, whose splendor the Lord spoke of, saying: "Then shall the righteous shine forth as the sun" (Matt. 13:43). It is because of this that the Kontakion of this Feast is said daily (when there is not a great feast) in the Service of the Typica in perpetual commemoration of the glory that will be the lot of the Saints. According to tradition, the Lord's Transfiguration came to pass forty days before His Crucifixion; this is why the Transfiguration is celebrated forty days before the Exaltation of the Cross."
(taken from: http://goarch.org/chapel/saints_view?contentid=155)
"Why did our Lord not manifest His divine glory on Tabor before all the disciples instead of before three of them? First, because He Himself gave the Law through the mouth of Moses: "At the mouth of two witnesses or at the mouth of three witnesses, shall the matter be established" (Deuteronomy 19:15). Therefore, three witnesses are sufficient. These three witnesses represent three main virtues: Peter Faith, for he was the first to confess his faith in Christ as the Son of God; James Hope, for, with faith in the promise of Christ, he was the first who laid down his life for the Lord, being the first to be slain by the Jews; John Love, for he reclined on the bosom of the Lord and remained beneath the Cross of the Lord until the end. God is not called the God of many but rather the God of the chosen. "I am the God of Abraham, the God of Isaac and the God of Jacob" (Exodus 3:6). God often valued a faithful man more than an entire nation. Thus, on many occasions, He wanted to destroy the entire Jewish nation, but because of the prayers of righteous Moses, spared that nation to live. God listened more to the faithful Prophet Elijah than to the entire unbelieving kingdom of Ahab. Because of the prayers of one man, God towns and people. Thus, the sinful town of Ustiug was to be destroyed by fire and hail had it not been saved by the prayers of the one and only righteous man in it, St. Procopius, the "fool for Christ" (July 8)."
You were transfigured on the Mount, Christ God revealing Your glory to Your disciples, insofar as they could comprehend. Illuminate us sinners also with Your everlasting light, through the intercessions of the Theotokos. Giver of light, glory to You.
You were transfigured upon the mount, O Christ our God, and Your disciples, in so far as they could bear, beheld Your glory. Thus, when they see You crucified, they may understand Your voluntary passion, and proclaim to the world that You are truly the effulgence of the Father.
Awake you dull thoughts, be not always groveling and bending my soul towards the earth; arise and go up to the height of divine ascent. Let us run to join Peter and Zebedeeʹs sons, and with them go directly to Mount Tabor; that with them we may see the glory of our God, and may also hear the voice that they heard from heaven; and they proclaimed the Father’s radiance.