And when the disciples heard it, they fell on their face, and were much afraid.
Read Chapter 17
Cyril of Alexandria
AD 444
Through their speaking together it shows that the old prophets also spoke the same things as Jesus, even if enigmatically. In great awe the disciples fell on their faces, and the Savior raised them up. This shows that if Jesus had not been incarnate and had not been Mediator between God and humanity and strengthened his own nature, he would not have endured to hear the voice of God.
And were very much afraid. There were two causes that might produce this fear in the apostles, the cloud that overshadowed them, or the voice of God the Father, which they heard. Their human weakness could not bear such refulgent beams of glory, and trembling in every limb, they fall prostrate on the ground. (St. Jerome)
The Almighty, it seems, was pleased to fulfil the wish of Peter, thereby to show that Himself is the tent or pavilion, under the shade of which the blessed shall live for ever, and to sanction the public and explicit confession of Peter relative to the divinity of Jesus Christ, by his own no less public and explicit confession, joined with an express command to hear and obey him. St. Chrysostom very justly remarks, that this voice was not heard till after the departure of Moses and Elias, that no possible doubt might exist to whom it was referred, and that it was to Christ only and to no other.
Hear ye Him: i.e. as the law and the prophets are fulfilled and verified ...
Why was it that when they heard these words they were dismayed? For previously a similar voice had spoken in Jordan and a crowd was present, but no one felt anything like this; and afterwards, again, when they said that it thundered, they did not feel anything like this. Why then did they fall on their faces on the mountain? Because there was solitude, and the height of the mountain, and great quietness, and a transfiguration full of awe, and a pure light and a cloud stretched out; all of these threw them into great alarm. Amazement arose on every side, and they fell on their faces in both fear and adoration at the same time. The Gospel of Matthew, Homily
How was it that, when they heard these words, they were dismayed? And yet before this also a like voice was uttered at Jordan, and a multitude was present, and no one felt anything of the kind; and afterwards again, when also they said, It thundered, John 12:28-29 yet neither at that time did they experience anything like this. How then did they fall down in the mount? Because there was solitude, and height, and great quietness, and a transfiguration full of awe, and a pure light, and a cloud stretched out; all which things put them in great alarm. And the amazement came thick on every side, and they fell down both in fear at once and in adoration.