From a homily by St Cyril of Alexandria
With three chosen disciples Jesus went up the mountain. Then he was transfigured by a wonderful light that made even his clothes seem to shine. Moses and Elijah stood by him and spoke with him of how he was going to complete his task on earth by dying in Jerusalem. In other words, they spoke of the mystery of his incarnation, and of his saving passion upon the cross. For the law of Moses and the teaching of the holy prophets clearly foreshadowed the mystery of Christ. The law portrayed it by types and symbols inscribed upon tablets. The prophets in many ways foretold that in his own time he would appear, clothed in human nature, and that for the salvation of all our race he would not refuse to suffer death upon the cross.
The presence of Moses and Elijah, and their speaking together, was meant to show unmistakably that the law and the prophets were the attendants of our Lord Jesus christ. He was their master, whom they had themselves pointed out in advance in prophetic words that proved their perfect harmony with one another. The message of the prophets was in no way at variance with the precepts of the law.
Moses and Elijah did not simply appear in silence; they spoke of how Jesus was to complete his task by dying in Jerusalem, they spoke of his passion and cross, and of the resurrection that would follow. Thinking no doubt that the time for the kingdom of God had already come, Peter would gladly have remained on the mountain. He suggested putting up up three tents, hardly knowing what he was saying. But it was not yet time for the end of the world; nor was it in this present time that the hopes of the saints would be fulfilled — those hopes founded on Paul’s promise that Christ would transform our lowly bodies into the likeness of his glorious body.
Only the initial stage of the divine plan had as yet been accomplished. Until its completion was it likely that Christ, who came on earth for love of the world, would give up his wish to die for it? For his submitting to death was the world’s salvation, and his resurrection was death’s destruction.
As well as the vision of Christ’s glory, wonderful beyond all description, something else occurred which was to serve as a vital confirmation, not only of the disciples’ faith, but of ours as well. From a cloud on high came the voice of God the Father saying: This is my beloved Son in whom I am well pleased. Listen to him.
St Cyril of Alexandria Homily 9 on the Transfiguration of the Lord. [PG 77: 1011-14]