When the fifth seal is opened John sees all the souls of those had been slain for the word of God gathered under the altar of God calling out for vengeance.
The altar (θυσιαστήριον) can refer to the altar in the court of the temple used for the daily sacrifices or the altar of incense inside the temple itself (Luke 1:11). But it can also refer to the sacrifice on the altar itself.
Who are these souls under the altar in Revelation 6? Are they just people killed in the tribulation or throughout the history of the church? Revelation refers to people killed for their testimony and their refusal to worship the beast (13:15) or refused his mark (20:4). The souls are under the altar because the resisted the kingdom of the beast. This could refer to all the martyrs for the whole history of the church (an idealist view) or just those who died in the tribulation (a futurist view). Beale suggests their location under the altar “emphasizes the divine protection that has held sway over their “soul” despite even their loss of physical life because of persecution” (Revelation, 392).
The people under the altar call out to God as “Sovereign Lord.” Lord is δεσπότης (despostes), a term that is normally used by a slave addressing their master, although it is used in the LXX 17 times for God. Aune points out it is a “regular Greek translation of two Latin terms for the Roman emperor” (Aune, Revelation 6–16, 407). This is a hint of the identity of the source of the persecution of God’s people in Revelation, the master who rules the world is not the real, “holy and true” master in heaven.
The people crying out are wearing white robes and are standing under the altar of God. To be under the altar is to be covered in the blood of the slain Lamb of God. In the seven letters, the ones who have overcome are given white robes (3:4-5; 18).
These souls ask God how long he will wait before avenging their deaths. The cry “how long?” appears in the Psalms and Zechariah 1:12. For example, Psalm 6:3, “my soul is greatly troubled, But you, O Lord, how long?” Psalm 13 begins with the words “How long, O LORD? Will you forget me forever? How long will you hide your face from me?” Psalm 74:10, “How long, O God, is the foe to scoff? Is the enemy to revile your name forever?”
The Lord tells them to rest a little longer until the full number of their brothers is complete. This answer can be taken several ways. First, it could refer to the total number of martyrs is reached. This implies God knows how many have been chosen to give their lives. Second, some would take this as a reference to when the last martyr dies, then Christ will return and destroy the kingdom of the Beast. The view of the early church was that God had established a “number” for the martyrs. A third possibility is this refers to the end of suffering on earth in general This may include martyrdom, but some will survive until the end to enter into the kingdom.
The souls call on God to avenge them. This is a common Old Testament theme: God is the avenger of the innocent. The souls under the altar are making a legal complaint to God for justice. Since God is the “just judge,” the martyrs can ask him to give them the justice they deserve by punishing the ones who put them to death.
Psalm 9:13 O Lord, see how my enemies persecute me! Have mercy and lift me up from the gates of death,
Fourth Ezra has a similar theme. In 4 Ezra 4:35-37 the souls of the righteous call out “how long” and look forward to the harvest when they would be rewarded. Like Revelation 6:10, they are told they must wait until “the number of those like yourselves is completed.”
4 Ezra 4:35–37 Did not the souls of the righteous in their chambers ask about these matters, saying, ‘How long are we to remain here? And when will the harvest of our reward come?’ 36 And the archangel Jeremiel answered and said, ‘When the number of those like yourselves is completed; for he has weighed the age in the balance, 37 and measured the times by measure, and numbered the times by number; and he will not move or arouse them until that measure is fulfilled.’ ”
In 1 Enoch 9:10 the ones who have died of blood and oppression bring a lawsuit to the gate of heaven, described as groaning under in the face of their oppression.
1 Enoch 9:10 And now behold, the Holy One will cry, and those who have died will bring their suit up to the gate of heaven. Their groaning has ascended (into heaven), but they could not get out from before the face of the oppression that is being wrought on earth
The fifth seal therefore vividly pictures those who have given their lives resisting the empire and holding on to their testimony for the Lord waiting on the Lord to avenge their deaths. The Lord’s words are comforting, they only need to wait a little while. The Lord will judge rightly between the oppressor and the oppressed and he will punish and reward with justice.