The fifth seal (Rev 6:9-11) speaks of souls under the altar crying for justice. The altar represented is the altar of sacrifice, which was symbolic of Jesus on the cross. Thus the souls are on earth. The cry for justice is a repeat of cry for justice long requested through the centuries, but more specifically it is the same cry as that made in Daniel 8:13-14. It is the cry for cleansing of God's name. The cry to make the injustice in the world right. I like David did not understand how the world's injustice will be remedied, until I looked into the sanctuary (Psalm 73).
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 witness they had borne. 10 They cried out with 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?” 11 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.
WHERE EXACTLY ARE THESE SAINTS - EARTH
1. What is the altar spoken of here?
There were two altars in the sanctuary. The altar of burnt offering and the altar of incense. Which one of the two mentioned involved blood at the base of the altar? Only the altar of burnt offering. (Exodus 29:12; Leviticus 4:18)
2. Where did the antitype of the altar of sacrifice occur?
Jesus fulfilled the symbolism of the altar of sacrifice by dying on the cross in Golgotha. Thus the location for this antitype would be earth. One other rite of the temple was fulfilled on earth and that was the laver, which was representative of Jesus' baptism.
3. Where did the antitype of the two compartments of the heavenly sanctuary take place?
Both compartments took place and are taking place in heaven. Thus the there is a distinctive dividing line between heaven and earth represented in the sanctuary.
4. Where are the souls under the altar?
If the altar is representative of the altar of sacrifice, then the souls are on earth. This is most interesting considering many attempts to insert the saints in heaven prior to the resurrection of Jesus.
What makes the temple so dirty?
Are these souls really talking?
5. Is Revelation 6:9-11 the only place where blood talks in the Bible?
The imagery here is similar to that used by the writer of Hebrews when speaking of Abel and Jesus (Hebrews 12:24).
6. What makes the cry of Jesus', Abel's, or the souls' blood under the altar so compelling?
It was unjust. Abel was killed by his jealous brother. The souls under the altar were killed for only witnessing, reading the Bible, going against the state religion, or some other good cause. Jesus was killed for sins he didn't commit.
7. What was it that caused the earthly temple to become unclean and in need of cleansing?
The unjust death of lambs. The lambs were killed for sins they did not commit. Each lamb's death was allowed by the temple servants, the temple constitution, and the God of the temple. Only the temple could stop the death of a lamb, but they never did for 2000+ years. Thus the temple was held culpable.
8. What was it that caused the heavenly temple to become unclean?
Jesus died for no sin he committed, but for the sins that we committed. The only person that could have stopped this death was God (Matt 26:39). Thus God and his house (temple) were held responsible for the death of Jesus and the filth of injustice settled on the temple.
9. Is the death of Jesus the only thing that causes the temple to be dirty?
No, when we become Christians we become the representatives of Christ (2 Cor 5:20). When Jesus sent out the twelve to go and do missionary work, he said, "Whoever receives you receives me, and whoever receives me receives him who sent me." (Matthew 10:40). Paul says that it is not he that lives, but Christ lives inside him (Galatians 2:20). The joy of the Christian is to follow in His master's steps (1 Peter 2:21-23; 4:12-19). It is still unjust. The fact that we are being killed and that we did the right thing doesn't erase the crime and just like in the case of Jesus' death the only being in the universe that could stop the injustice is God. Thus God is held culpable and his house/temple is soiled with injustice.
The same question in Daniel
10. Does the question of how long occur elsewhere in the Bible?
Yes, 60 times in the KJV. Psalm 74:10 states, "O God, how long shall the adversary reproach? shall the enemy blaspheme thy name for ever?".
11. Is there a verse or verses in the context of the sanctuary and apocalyptic prophecy that deals with the same question?
Yes, Daniel 8:13-14 and Daniel 12:6. Here is what Daniel 8:13-14 says.
Then I heard a holy one speaking; and [another] holy one said to that certain [one] who was speaking, "How long [will] the vision [be, concerning] the daily [sacrifices] and the transgression of desolation, the giving of both the sanctuary and the host to be trampled underfoot? And he said to me, "For two thousand three hundred days; [fn] then the sanctuary shall be cleansed."
12. What started to happen on 1844?
God started to get things in order so that he could fix the wrongs that were committed against Jesus and all of those that died representing Jesus through the centuries. It is the preadvent judgement. While not all have kiled God's saints, all the earth is held guilty for Jesus' death. It was our sin that put him there, because we should have died long ago, but we are still living. Every breath we breath is an injustice a stain on the heavenly temple. Jesus said, And the one who falls on this stone will be broken to pieces; and when it falls on anyone, it will crush him." (Matt 21:44). The only way to fix our problem, is to believe in the one we killed.
13. When will Jesus make his killers/the world pay for their wrongdoing?
Behold, he is coming with the clouds, and every eye will see him, even those who pierced him, and all tribes of the earth will wail on account of him. Even so. Amen (Rev 1:7).