We need to dig deeper into the minds of these end-time spelunkers. What thinking underlies their words?
A shaking fist is absent; these individuals do not express anger or outright rebellion against God. Conversely, they make no confession of personal guilt; they express no repentance. While they recognize the existence of the Father and Son, they do not understand that God is a Family into which they can be born. They do not know—or believe—the gospel. They do not realize that they can develop a personal relationship with God and grow to become like Him. In other words, the cavemen's words are not those of converted individuals at all.
The underlying thinking behind their comments is desperate self-preservation.
They want personal safety. Understanding more than many do about God, convinced that the Father and the Lamb are stirred to anger, their knowledge is still so limited that they can only irrationally command "mountains and rocks" to fall on them. Pathetically, in the end, they can only ask a question that exhibits the depths of their despair. Who is able to survive during the Day of the Lord? They have no answer.
Isaiah 2 provides us a bit more insight. In verse 9, the prophet, speaking of idolaters, addresses the issue of their repentance. These people, he says, "will be brought low and everyone humbled—do not forgive them" (Isaiah 2:9, New International Version). God has humbled them through mind-numbing terror; they hide in caves from God and His Son and talk to rocks. Yet, in all this, they have not yet expressed godly sorrow, not yet repented. So God has not yet forgiven them. The prophet Isaiah continues:
Enter into the rock, and hide in the dust, from the terror of the LORD and the glory of His majesty. The lofty looks of man shall be humbled, the haughtiness of men shall be bowed down, and the LORD alone shall be exalted in that day. For the day of the LORD of hosts shall come upon everything proud and lofty, upon everything lifted up—and it shall be brought low. . . . (Isaiah 2:10-12)
Notice that the cave-dwellers are those who have been humbled. In verse 11, Isaiah states the timeframe: They are humbled in a time when "the LORD alone shall be exalted. . . ." So, this passage in Isaiah 2 is dealing with the general period that we call the Day of the Lord.
Interestingly, in verses 20-21, we see that they have eschewed idolatry:
In that day a man will cast away his idols of silver and his idols of gold, which they made, each for himself to worship, to the moles and bats, to go into the clefts of the rocks, and into the crags of the rugged rocks, from the terror of the LORD and the glory of His majesty, when He arises to shake the earth mightily.
If the people who hurl their idols "to the moles and bats" as they enter the "clefts of the rocks" are the same ones who ask "mountains and rocks" to fall on them in Revelation 6:16, these folk may well have started out on a road to repentance. They are not there yet, for they lack the proper understanding and motivations. Though God has not yet granted them repentance (II Timothy 2:25), He is working among them, perhaps through the work of the Two Witnesses. He has increased their knowledge about Him, brought them to an understanding that idolatry is wrong, and led them to subterranean "places of safety."