John Calvin Commentary Matthew 17:6

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

And having heard this, God intended that the disciples should be struck with this terror, 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 God, or despise Him without concern, it is because God does not address them so that His presence is felt; but the majesty of God, as soon as we perceive Him, must unavoidably cast us down.