"AND in this Transfiguration the foremost object was to remove the offense of the cross from the disciple's heart, and to prevent their faith being disturbed by the humiliation of His voluntary Passion by revealing to them the excellence of His hidden dignity. But with no less foresight, the foundation was laid of the Holy Church's hope, that the whole body of Christ might realize the character of the change which it would have to receive, and that the members might promise themselves a share in that honour which had already shone forth in their Head. About which the Lord had Himself said, when He spoke of the majesty of His coming, "Then shall the righteous shine as the sun in their Father's Kingdom" (Matt 13:43), while the blessed Apostle Paul bears witness to the self-same thing, and says: "for I reckon that the sufferings of this time are not worthy to be compared with the future glory which shall be revealed in us" (Rom 8:18): and again, "for you are dead, and your life is hid with Christ in God. For when Christ our life shall appear, then shall you also appear with Him in glory" (Col 3:3). But to confirm the Apostles and assist them to all knowledge, still further instruction was conveyed by that miracle."
~St. Leo the Great: Sermon on the Transfiguration.
Read the complete sermon here