And it came to pass, about eight days after these words, that he took Peter and James and John and went up into a mountain to pray. And whilst he prayed, the shape of his countenance was altered and his raiment became white and glittering. And behold two men were talking with him. And they were Moses and Elias, Appearing in majesty. And they spoke of his decease that he should accomplish in Jerusalem. But Peter and they that were with him were heavy with sleep. And waking, they saw his glory and the two men that stood with him. And it came to pass that, as they were departing from him, Peter saith to Jesus: "Master, it is good for us to be here: and let us make three tabernacles, one for thee, and one for Moses; and one for Elias": not knowing what he said. And as he spoke these things, there came a cloud and overshadowed them. And they were afraid when they entered into the cloud. And a voice came out of the cloud; saying: "This is my beloved son. Hear him." And whilst the voice was uttered Jesus was found alone. And they held their peace and told no man in those days any of these things which they had seen. (Luke 9:28-36)
THABOR AND GOLGOTHA: This glimpse of glory was meant to make such an impression on the three apostles, as to prevent their losing courage or faith when, ere long, they saw their Lord in the hour of His deep abasement, and in that fearful state of suffering, when "there was no beauty or comeliness in Him". In fact, the Transfiguration contrasts with the Crucifixion in every respect. In the one, we perceive Christ in wondrous majesty--on either side of Him two Saints--, the revelation of God, and the desciples in rapture. In the other we see our Lord marred and disfigured--on either side of Him two thieves--, abandoned by God and with Him His sorrowing Mother, the grief-stricken John, and the weeping women.
[From 'A Practical Commentary on Holy Scripture' by Bishop Knecht, D.D.]
(1899 Douay-Rheims Bible)