O God, who on the holy mount didst reveal to chosen witnesses Thy well-beloved Son, wonderfully transfigured, in raiment white and glistening: Mercifully grant that we, being delivered from the disquietude of this world, may by faith behold the King in his beauty; who with thee, O Father, and thee, O Holy Ghost, liveth and reigneth, one God, world without end.The feast of the Transfiguration of Christ celebrates the revelation of Christ's divine glory on Mount Tabor in Galilee. After revealing to His disciples that He would be put to death in Jerusalem (Matthew 16:21), Christ, along with Ss. Peter, James, and John, went up the mountain. There, St. Matthew writes, "he was transfigured before them. And his face did shine as the sun: and his garments became white as snow."
The brightness was not something added to Christ but the manifestation of His true divine nature. For Peter, James, and John, it was also a glimpse of the glories of heaven and of the resurrected body promised to all Christians. As Christ was transfigured, two others appeared with Him: Moses, representing the Old Testament Law, and Elijah, representing the prophets. Thus Christ, Who stood between the two and spoke with them, appeared to the disciples as the fulfillment of both the Law and the prophets.
At Christ's baptism in the Jordan, the voice of God the Father was heard to proclaim that "This is my beloved Son" (Matthew 3:17). During the Transfiguration, God the Father pronounced the same words (Matthew 17:5).