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,.... 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.

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 Mt 2:11.

17:6 They... were sore afraid. Like the children of Israel at Sinai, they were filled with awe at the Divine voice (Ex 19:9,16).

17:1-13 Now the disciples beheld somewhat of Christ's glory, as of the only begotten of the Father. It was intended to support their faith, when they would have to witness his crucifixion; and would give them an idea of the glory prepared for them, when changed by his power and made like him. The apostles were overcome by the glorious sight. Peter thought that it was most desirable to continue there, and to go no more down to meet the sufferings of which he was so unwilling to hear. In this he knew not what he said. We are wrong, if we look for a heaven here upon earth. Whatever tabernacles we propose to make for ourselves in this world, we must always remember to ask Christ's leave. That sacrifice was not yet offered, without which the souls of sinful men could not have been saved; and important services were to be done by Peter and his brethren. While Peter spoke, a bright cloud overshadowed them, an emblem of the Divine presence and glory. Ever since man sinned, and heard God's voice in the garden, unusual appearances of God have been terrible to man. They fell prostrate to the earth, till Jesus encouraged them; when looking round, they beheld only their Lord as they commonly saw him. We must pass through varied experiences in our way to glory; and when we return to the world after an ordinance, it must be our care to take Christ with us, and then it may be our comfort that he is with us.