Fifth Seal:

9 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 testimony which they held. 10 And they cried with a loud voice, saying, “How long, O Lord, holy and true, until You judge and avenge our blood on those who dwell on the earth?” 11 Then a white robe was given to each of them; and it was said to them that they should rest a little while longer, until both the number of their fellow servants and their brethren, who would be killed as they were, was completed.

Commentary:

This is the first seal opened that is not followed by a horseman. This seal gives us a glimpse into heaven where martyrs of the faith are being comforted as they cry for justice. John records that under the altar (Lev 17:11) are those who had been slain (a) for the word of God and (b) for the testimony they held. These were faithful Christians persecuted and ultimately murdered for their faith. The word 'earth' (gé) can be translated land and I believe is referring to the inhabitant, specifically the persecutors of these saints, namely those persecutors in Jerusalem. I think this supports John describing the martyrs as under the altar. This imagery would point a finger at those in the 'earth' (land/Jerusalem) who make sacrifices- the priests.

This harmonizes with the words of Christ in Matthew 23:34-37:

34 Therefore, indeed, I send you prophets, wise men, and scribes: some of them you will kill and crucify, and some of them you will scourge in your synagogues and persecute from city to city, 35 that on you may come all the righteous blood shed on the earth, from the blood of righteous Abel to the blood of Zechariah, son of Berechiah, whom you murdered between the temple and the altar. 36 Assuredly, I say to you, all these things will come upon this generation. (NKJV)

Just as the religious leaders in Jesus day murdered Him out of jealousy and envy, so were these faithful followers of Christ murdered. And in heaven, they ask a legitimate question, 'How long' in which they are clothed in white robes and told that the persecution would continue until the full number of those God ordained to be martyred were complete. In other words the religious leaders of Jerusalem had yet to max out their iniquity. Once they did, God would execute his justice, and he did that in 70 AD when he destroyed their temple.

Meditation Point:

Our Triune God destroyed the temple of the religious leaders that persecuted Jesus and his followers sending a clear message to them that (a) Jesus is God, (b) His followers are His bride and (c) He is a God of justice.