6:15 And the kings of the earth and the great men and the commanders and the rich and the strong and every slave and free man, hid themselves in the caves and among the rocks of the mountains;
HID THEMSELVES in the caves and among the rocks of the mountains;
So great is the catastrophe that ALL mankind, leaders, wealthy, and just plain ordinary folks will want to hide from the great COSMIC display of wrath. Not even the false security of one's position, Isaiah 47:8-11, will stop this display of wrath.
It is interesting to note that there is no repentance in these peoples wish; they just want to hide. This would indicate that they have made the choice to NOT follow Jesus. And if there are believers during this terrible event, they can be assured that God is still the same (Psalm 102:23-28). The earth may wear out as a garment, Isaiah 51:4-6-8, but 'My salvation is forever'. This event will occur as prophesied, Isaiah 13:9-13.
Be aware, though, this is not just a terrible event; it is testing of things which can be shaken. In Hebrews 12:18-28-29, the things which CANNOT be shaken are those which matter. They are an acceptable service with reverence and awe.