When he opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the witness they had borne. They cried out with a loud voice, “O Sovereign Lord, holy and true, how long before you will judge and avenge our blood on those who dwell on the earth?” Then they were each given a white robe and told to rest a little longer, until the number of their fellow servants and their brothers should be complete, who were to be killed as they themselves had been. (Revelation 6:9-11, ESV)
The first four seals focused on the earth and events on the earth. The fifth seal looks at heaven. We see an alter. Alters, like this one, were associated with the tabernacle and temple. In the temple, there were two alters, the alter in the outer courtyard that was used for sacrificing animals to atone and cover sin, and an alter in the Holy Place just before the curtain of the Holy of Holies. This last alter was the incense alter. The burned incense represented the prayers of the people. The one alter in Revelation appears to serve both purposes the two alters in the temple.
In this passage the alter as the alter of sacrifice. John sees "those who had been slain for the word of God and for the witness they had borne" under the alter. Under the alter was the place where the blood of the sacrificed animals would be poured out. Those under the alter are the souls of those suffered and died for Christ. Their blood had been sacrificially poured out in their suffering and death. They are without physical bodies, they live in heaven in the period after Christ's first advent and before his second coming. It is an indication that those who die in Christ will consciously live in heaven with God. They will be aware of what is happening on earth. They remember how the earthly "courts" condemned them to death and ask the highest judge of the universe to vindicate them.
In a sense, all who belong to Christ, even on earth, groan for the final redemption when all things are put right. We read in Romans:
For the creation waits with eager longing for the revealing of the sons of God. For the creation was subjected to futility, not willingly, but because of him who subjected it, in hope that the creation itself will be set free from its bondage to corruption and obtain the freedom of the glory of the children of God. For we know that the whole creation has been groaning together in the pains of childbirth until now. And not only the creation, but we ourselves, who have the firstfruits of the Spirit, groan inwardly as we wait eagerly for adoption as sons, the redemption of our bodies. For in this hope we were saved. Now hope that is seen is not hope. For who hopes for what he sees? But if we hope for what we do not see, we wait for it with patience. (Romans 8:19-25, ESV)
How long must they wait before Christ returns to set things aright? The answer elsewhere in scripture is that Christ will not return until the gospel, the good news of Christ, goes throughout the nations. The flip side of the coin is shown in this passage in Revelation, Christ is waiting until the rest of the designated number of the saints on earth are slain. The gospel comes with a price for going throughout the world. First and foremost, Christ was slain for the sins of the world which is the foundation for the gospel. But the gospel also goes forth throughout the centuries through the suffering and blood of the saints.
God comforts those who have been slain. God grants them a white robe, rest from their toil, and a promise they will be vindicated. The white robe is God's recognition of their purity, their victory, having suffered for Christ. But ultimately the white robes come from Christ's sacrifice on their behalf. The saints in heaven before Christ's second coming enjoy the peace and rest from their labors. They see the face of God and enjoy his presence. They see God hearing their prayers and responding to them and all the saints on earth.
The fifth seal is a pivotal seal in the events in heaven and earth. We will see God responding to their plea in the rest of Revelation.
Are you caught in difficulty and suffering because of Christ? Do you seem to be at wit's end? Are you suffering because of this fallen world? Death, disease, and toil all result from the fall. The whole of creation is groaning from the fall of Adam. The fifth seal and the martyrs' death represents the suffering for all who persevere through suffering. We have a God who cares, who hears our cries for help, and responds. For our ultimate vindication, we will need to wait until all the appointed suffering is complete, which is intimately linked to the spread of the gospel throughout the nations. If you are a true follower of Christ, God will grant you strength to persevere until death, and through death you will gain your ultimate victory of the white robe of purity and conquest, and you will enjoy the rest of being in the presence of God.