He used this transfiguration chiefly that the scandal of the cross would be lifted from the hearts of the disciples and that the humility of his voluntary suffering would not upset the faith of those to whom the perfection of his hidden dignity had been revealed. But with no less foresight, the hope of the holy church was made firm, so that it might know with what sort of exchange the whole body of Christ was to be given and that the members might promise to themselves a sharing in the honor of the one who had shone as their Head.
The Lord had said, when speaking about the majesty of his coming, "Then the just will shine like the sun in the kingdom of their Father" (Matt. 13:43). Blessed Paul the apostle assures us the same thing when he says, "For I think that the sufferings of this time are not to be compared with the future glory that will be revealed to us." On another occasion the same apostle said, "For you have died, and your life is hidden with Christ in God. And when Christ your life appears, you also will appear with him in glory" (Colossians 3:3). To strengthen the apostles and to advance them in all knowledge, yet another lesson came in this miracle. Moses and Elijah, meaning the law and the prophets, appeared speaking with the Lord, so that very truly in the presence of these five men, what was written might be fulfilled: "Among two or three witnesses, every word will stand." What is more stable, what is more firm, than this saying in whose message the trumpets of the Old and of the New Testaments sound and with whose gospel teaching the records of the ancient pronouncements concur? The pages of both Testaments agree with one another, and the splendor of his present glory shows, manifest and clear. the one whom the preceding signs had promised under a veil of mystery. As blessed John says, "The Law was given through Moses; grace and truth have come through Jesus Christ" (John 1:17) Sermon 51.2-4" pg. 22 "We Believe in the Crucified and Risen Lord" Vol. 3 [Ancient Christian Doctrine]
" And after six days Jesus taketh Peter, James, and John his brother, and bringeth them up into an high mountain apart,
 And was transfigured before them: and his face did shine as the sun, and his raiment was white as the light.
 And, behold, there appeared unto them Moses and Elias talking with him.
 Then answered Peter, and said unto Jesus, Lord, it is good for us to be here: if thou wilt, let us make here three tabernacles; one for thee, and one for Moses, and one for Elias.
 While he yet spake, behold, a bright cloud overshadowed them: and behold a voice out of the cloud, which said, This is my beloved Son, in whom I am well pleased; hear ye him.
 And when the disciples heard it, they fell on their face, and were sore afraid.
 And Jesus came and touched them, and said, Arise, and be not afraid.
 And when they had lifted up their eyes, they saw no man, save Jesus only.
 And as they came down from the mountain, Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead." Matthew 17:1-9