Juxtaposed against "the kings of the earth, the great men, the rich men, the commanders, the mighty men" is another group: "every slave and every free man" (Revelation 6:15). Who are they? What role do they play in the caves?
To understand, we first need to deal with those repeated words, every: "every slave and every free man." Does John mean that every slave and every free person in the world is addressing "mountains and rocks," asking that they fall on him? Does every free individual and every slave know about the Day of the Lord and about the Lamb at this point? That would be a lot of people.
Revelation 9 clearly indicates that the cave-dwellers represent only a segment—perhaps a small segment—of humanity. Many other people have refused to foreswear idolatry, not yet understanding what the cavemen know about God and His imminent anger:
But the rest of mankind, who were not killed by these plagues, did not repent of the works of their hands, that they should not worship demons, and idols of gold, silver, brass, stone, and wood, which can neither see nor hear nor walk. And they did not repent of their murders or their sorceries or their sexual immorality or their thefts. (Revelation 9:20-21)
So, the occurrences of "every" in Revelation 6:15 do not refer to every slave and every free person in the world. Rather, the phrase "every slave and every free man" is a merism, a rhetorical device wherein a single entity or action is described by opposites, as in "looked high and low" or "on-and-off enthusiasm." "Every slave and every free man" refers to a small subset of people, to a single class of person, one who is both free and bond.
The merism may refer to God's people—who are free and slave concurrently. Christ promises that, if we remain in His Word, we are free: "If you abide in My word, you are My disciples indeed. And you shall know the truth, and the truth shall make you free" (John 8:32). Similarly, the apostle Paul writes:
There is therefore now no condemnation to those who are in Christ Jesus, who do not walk according to the flesh, but according to the Spirit. For the law of the Spirit of life in Christ Jesus has made me free from the law of sin and death. (Romans 8:1-2; compare Galatians 5:1)
Yet, the same apostle calls us slaves, bought by God:
Or do you not know that your body is the temple of the Holy Spirit who is in you, whom you have from God, and you are not your own? For you were bought at a price; therefore glorify God in your body and in your spirit, which are God's. (I Corinthians 6:19-20)
Paul also tells the Roman church: "But now having been set free from sin, and having become slaves of God, you have your fruit to holiness, and the end, everlasting life" (Romans 6:22). Peter provides yet further witness to our being God's slaves: "For this is the will of God, that by doing good you may put to silence the ignorance of foolish men—as free, yet not using liberty as a cloak for vice, but as bondservants of God" (I Peter 2:15-16).
In some ways, God's people are free, and in others, slaves.
We could look at this merism a bit differently. "Every slave and every free man" could refer to true Christians, those who know the truth and are therefore free (John 8:32) in God's sight, but who have become enslaved by man through end-time religious persecution. Slaves are expropriated and disenfranchised individuals, having lost personal and property rights. The Jews, taken in the Nazi pogroms, were slaves, told by their masters, "Arbeit macht frei" ("Work makes free").
Currently, chattel slavery is not a legal institution in Western civilization. However, under increased Islamic influence, it could become legalized and widespread as the result of religious persecution. So it might happen that God protects His people in caves, arranging to have them taken there as slaves in service to others.