They fell on their face - They entered into the cloud, or the cloud enveloped them, Luke 9:34. "They were therefore afraid." They were awed at the presence of God, and prostrated themselves in solemn adoration on the ground, and their fears were removed only by the voice of their beloved Master. No man can see God and live; and it is only the glory of God, as it shines in the face of Christ (see 2 Corinthians 4:6), that mortals can bear.

Matthew 17:6 Parallel Commentaries

On the Words of the Gospel, Matt. xvii. 1, "After Six Days Jesus Taketh with Him Peter, and James, and John his Brother," Etc.
1. We must now look into and treat of that vision which the Lord showed on the mount. For it is this of which He had said, "Verily I say unto you, there be some standing here which shall not taste of death till they see the Son of Man in His Kingdom." [2643] Then began the passage which has just been read. "When He had said this, after six days He took three disciples, Peter, and James, and John, and went up into a mountain." [2644] These three were those "some," of whom He had said, "There be some …
Saint Augustine—sermons on selected lessons of the new testament

The Last Events in Galilee - the Tribute-Money, the Dispute by the Way, the Forbidding of Him who could not Follow with the Disciples, and The
Now that the Lord's retreat in the utmost borders of the land, at Cæsarea Philippi, was known to the Scribes, and that He was again surrounded and followed by the multitude, there could be no further object in His retirement. Indeed, the time was coming that He should meet that for which He had been, and was still, preparing the minds of His disciples - His Decease at Jerusalem. Accordingly, we find Him once more with His disciples in Galilee - not to abide there, [3743] nor to traverse it …
Alfred Edersheim—The Life and Times of Jesus the Messiah

On the Morrow of the Transfiguration
IT was the early dawn of another summer's day when the Master and His disciples turned their steps once more towards the plain. They had seen His Glory; they had had the most solemn witness which, as Jews, the could have; and they had gained a new knowledge of the Old Testament. It all bore reference to the Christ, and it spake of His Decease. Perhaps on that morning better than in the previous night did they realise the vision, and feel its calm happiness. It was to their souls like the morning-air …
Alfred Edersheim—The Life and Times of Jesus the Messiah