John Calvin Commentary Isaiah 2:21

"to go into the caverns of the rocks, and into the clefts of the ragged rocks, from before the terror of Jehovah, and from the glory of his majesty, when he ariseth to shake mightily the earth." — Isaiah 2:21 (ASV)

And they shall enter into the clefts of the rocks—this repetition is not superfluous, though Isaiah again employs the same words he had recently used. For what is so difficult as to impress upon people's minds sincere fear of God?

Nor do we perceive this only in hypocrites, but also in ourselves, if we pay careful attention. For how many things are presented to us that should deeply affect our minds, and yet we are hardly moved!

Especially, it was necessary that this judgment of God should be earnestly set before hypocrites, who delighted in wickedness.

But now he points out the severity of God’s vengeance by this: that the ungodly would rather be swallowed up by the deepest gulfs than come under the eye of God.

This, too, is the passage from which Christ borrowed the threatening he pronounces against the Jews:

In that day shall they say to the mountains cover us; and to the hills, Hide us. (Luke 23:30)