And having heard this God intended that the disciples should be struck with this terror, in order to impress more fully on their hearts the remembrance of the vision. Yet we see how great is the weakness of our nature, which trembles in this manner at hearing the voice of God. If ungodly men mock at God, or despise him without concern, it is because God does not address them so as to cause his presence to be felt; but the majesty of God, as soon as we perceive him, must unavoidably cast us down.

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.

Fell on their face - Dismayed by the voice, and dazzled by the glory of the cloud. So Daniel, Daniel 8:17, and Saul of Tarsus, Acts 9:4.

And when the disciples heard [it], they (d) fell on their face, and were sore afraid.
(d) Fell down flat on their faces and worshipped him, as in (Matthew 2:11).

And when the disciples heard it,.... The voice out of the cloud, and which they apprehended came from God, and was uttered with so much majesty:
they fell on their face: not so much out of reverence, or for the sake of adoration, but as persons struck with astonishment and fear, and were as half dead; and so fell with their faces fiat to the ground, not being able to stand before God, to behold his majesty, and hear his voice:
and were sore afraid: they were filled with fear, when, awaking out of their sleep, they saw the surprising glory of Christ, and of the two men that were with him, insomuch that they knew not what to think, or say; and so they were when they entered into the cloud, and still more upon hearing the voice of God himself, even though it was a voice of love, grace, and mercy; see Deuteronomy 5:24. But yet they were not struck with so much amazement and surprise, as not to know what was said; for they distinctly heard the words, rightly understood, and faithfully related them; from whom the evangelists had them, and which Peter perfectly remembered, and recorded many years after.

They were sore afraid. Like the children of Israel at Sinai, they were filled with awe at the Divine voice.