"Jesus took Peter and James and his brother John, and ascending a high mountain with them, showed them the brightness of his glory; for, although they had recognized the majesty of God in him, yet they did not know the power of his body, wherein his deity was contained. This is why the Lord had promised that some of his disciples should not taste death till they saw "the Son of man coming in his Kingdom”, that is, in the kingly brilliance which specially belonging to the human nature he had assumed…
The great reason for this transfiguration was to remove the scandal of the cross from the hearts of his disciples, and to prevent the humiliation of his voluntary suffering from disturbing the faith of those who had witnessed the surpassing glory that lay concealed. With no less forethought he was also providing a firm foundation for the hope of holy Church. The whole body of Christ was to understand the kind of transformation that it would receive as his gift. The members of that body were to look forward to a share in that glory which first blazed out in Christ their head.
The Lord had himself spoken of this when he foretold the splendour of his coming: “Then the just will shine like the sun in the kingdom of their Father” (Mt 13:43). Saint Paul the apostle bore witness to this same truth when he said, “I consider that the sufferings of the present time are not to be compared to the future glory that is to be revealed in us” (Rm 8,18). In another place he says, “You are dead, and your life is hidden with Christ in God. When Christ, your life, is revealed, then you also will be revealed with him in glory” (Col 3:4)."
-Saint Leo the Great