Albert Barnes Commentary Matthew 17:6

"And when the disciples heard it, they fell on their face, and were sore afraid." — Matthew 17:6 (ASV)

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 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.