They cried in a loud voice, "O [Sovereign] Lord, holy and true, how long now before You will sit in judgment and avenge our blood upon those who dwell on the earth?"
If we are any kind of serious student of the Bible, this verse will pierce our hearts. It is a picture of the martyrs for Christ crying out for His righteous indignation over their physical deaths at the hands of God-haters. Notice, I said physical deaths, because the persecutors of God's own can only kill the body, and after that, there is no more that they can do; our spirits live.
And I believe that the spirits of these holy martyrs now exist very near to Christ in Heaven and are mentioned in various translations as being "at the foot of the altar" or "under the altar". Now, I have read various commentaries that attempt to explain that this is symbolic because there is no way that there could be an altar big enough to accommodate all the Christian martyrs throughout time. But that strikes me as limiting God and His creation of Heaven. Don't you think He can make it as big as He wants?
I also believe that God's Temple on earth is an exact replica of His Temple that is in Heaven, and the Throne Room may be the Holy of Holies in the earthly model. Granted, I can't prove any of that, and to be honest, it doesn't really matter to me because the point of this Scripture is not to determine what or where the altar is, but to recognize, first, the reason that these martyrs suffered. It was because they believed the Word of God, and they declared the truth of it, and they boldly confessed their faith by holding fast to it without wavering, even though they died for it.
The second point of this verse is that these martyrs expect Christ to avenge their deaths. They expect Jesus to impart a righteous response to their persecution for the honor of God and Himself; for the Gospel Message that they were willing to die for; and for the holy terror and persuasion of others that this will be a part of their experience as Evil continues to march towards the final battle.
Also, notice that they do not ask God and Jesus to give them the means to avenge themselves. They know that God will take a just revenge upon the sin of persecution, and they leave it to Him to execute. They also know, as Matthew Henry explains so well, "There is a number of Christians, known to God, who are appointed as sheep for the slaughter, set apart to be God’s witnesses. As the measure of the sin of persecutors is filling up, so is the number of the persecuted martyred servants of Christ. When the number of martyrs is fulfilled, God will take a just and glorious revenge upon their cruel persecutors; he will bring tribulation to those who have spilled the blood of martyrs.
Down through the centuries, Christians have pictured those martyred spirits as coming from their Age. And so, we, too, can imagine the spirits of the men, women, and children who are dying for Christ in the Middle East; imagine them crying out from the altar at the feet of Jesus. And there are martyrs who will give their lives from our land, and we must prepare ourselves to be as convicted and steadfast as those faithful who have preceded us.
Sadly, there are too many Christians in America who think that Revelation 6:10 could never apply to them. They only see Jesus as the gentle Lamb. They cannot conceive Him as being angry or that the wrath of the Lamb will be exceedingly dreadful. They cannot imagine that it might be their blood that cries out for vengeance. Are we getting close to that final number of martyrs? It sure feels like there is an acceleration, doesn't it? This much I know ... we have had our season of Grace, and the Bible tells us that God will have His day of righteous wrath. When that day comes, the most evil sinners will not be able to stand before Him. But the question we all need to ask ourselves is this: Am I ready to stand for Him?