Luke 9:32

But Peter and they that were with Him were heavy with sleep; and when they were awake, they saw His glory, and the two men that stood with Him.

After these things had happened, after Peter had spoken the confession in the name of all the disciples, a matter of about eight days, on the eighth day after, Jesus took Peter and John and James along with Him. He wanted to give them visual evidence and proof that He was truly the Son of the living God. He ascended the mountain with them, the highest mountain in the neighborhood where they were at that time, a mountain well known to all of them. The Lord's purpose was to pray, to enter into intimate communion with His heavenly Father, for the purpose of getting wisdom and strength for His coming difficult work, for the Galilean ministry was drawing to a close, and the days of the Judean ministry would be short. And God revealed Himself in a remarkable manner to His Son. For while Jesus was engaged in prayer, His entire aspect changed. The appearance of His face became unlike His usual self, and all His clothing became white and resplendent, shining, flashing like lightning. And suddenly there were two men that appeared and were engaged in conversation with the Lord, namely, Moses and Elijah. In the case of the first, only God knew his grave, and as for the second, the Lord took him up to heaven outright. Moses had given the Law and was the great exponent of the Old Testament covenant, and Elijah had been zealous for the Law and suffered much for his faithfulness. Both of them had looked forward with eager longing to the coming of the Messiah. And now that the Christ had appeared on earth and was engaged in^ the work of His ministry, God permitted and caused these men to appear to Jesus on the mountain before the amazed eyes of the three apostles. Thus Peter and the others were witnesses of the glory of Jesus, 2 Peter 1:16. The divine glory, which He otherwise bore hidden before the eyes of men and only occasionally made manifest in word and deed, this glory now shone forth through His weak flesh, imparting to it that wonderful majesty which it was destined to bear at all times after entering into the final glory. Meanwhile, Peter and the other men were almost overwhelmed by the glory of the revelation; the brightness and the wonder of it all affected them so that they were as if heavy with sleep; they could barely manage to open their eyes from time to time. They heard only that Moses and Elijah were conversing with Jesus concerning His going out of this life, concerning the consummation of His ministry, which was to be fulfilled at Jerusalem and take place through suffering and death. And sometimes, when they roused themselves for a few moments, the disciples caught sight of the glory of their Master and of the two prophets that were standing with Him.