For we did not follow cleverly devised myths when we made known to you the power and coming of our Lord Jesus Christ, but we were eyewitnesses of his majesty. For when he received honor and glory from God the Father, and the voice was borne to him by the Majestic Glory, "This is my beloved Son, with whom I am well pleased," we ourselves heard this very voice borne from heaven, for we were with him on the holy mountain. And we have the prophetic word more fully confirmed, to which you will do well to pay attention as to a lamp shining in a dark place, until the day dawns and the morning star rises in your hearts, knowing this first of all, that no prophecy of Scripture comes from someone's own interpretation. For no prophecy was ever produced by the will of man, but men spoke from God as they were carried along by the Holy Spirit.
2 Peter 1:16-21
The reality of the eternal world is underlined by the apostles' preaching of the coming again of our Lord in power and glory, and Peter emphasises that the certainty of that glorious prospect is proved, first, by the Transfiguration, which was an anticipation, a foretaste, of His coming glory, and, second, by the utterances of the prophets who predicted it. We did not preach, says the Apostle, as those that had concocted some fantastic tale about the return of Christ in glory; we saw His glory, we had a foretaste, a blessed and glorious glimpse, of what it will be like on that great Day. How could we doubt His return in glory, we who have already been eye-witnesses of His majesty? This is one of the passages of Scripture which give an unerring indication of how a great biblical theme should be interpreted and understood. Whatever else we may take out of the story of the Transfiguration we may take it as authoritative that it was an earnest of His second Advent. On the mount, three men only viewed His kingly radiance, but when He comes to reign, every eye shall see Him, and amid the heavens bursting into light in the blaze of His glory a voice will be heard, like the sound of many waters, a voice that will be the delight or the terror of the universe, saying, 'This is My beloved Son, in Whom I am well pleased'. And we shall see His face. And we shall say, too this is our God, we have waited for Him: Glorious, blessed prospect! Even so, come Lord Jesus!