Does Revelation 6:9 establish the destination of the martyred souls?
Answer: The martyred souls "under the altar" symbolize all saints whose destination is Heaven.
Revelation 6:9: “[I] saw underneath the altar the souls of those who had been slain because of the word of God, and because of the testimony which they had maintained; and they cried out with a loud voice, saying, 'How long, O Lord, holy and true, will You refrain from judging and avenging our blood on those who dwell on the earth?'
11And there was given to each of them a white robe; and they were told that they should rest for a little while longer, until the number of their fellow servants and their brethren who were to be killed even as they had been, would be completed also.”
We, the faithful, are those suffering souls! The passage is describing the faithful of God who have (and are) cried out to Him (whether martyred or not).
God’s saints have been persecuted (some beheaded or worse) throughout history.
Which one of us has not watched the evening news and put our head in our hands asking: “How long Lord, before you avenge the injustices on the earth -- how long before it all ends?”
Next, we are told how these souls were given a white robe (metaphorically, we must persist in Christ's commandments) as we persevere before the full compliment of our fellow servants is complete. It is at that point, when there are none left to save where God will bring the world to an end. This is precisely the same as in the days of Noah, where God patiently waited (a century) for the pre-Flood population to come to repentance (none did).
Recall what is related much later in the Book of Revelation (chapter 20) to understand why:
Revelation 20:4: “Then I saw thrones, and they sat on them, and judgment was given to them. And I saw the souls of those who had been beheaded because of their testimony of Jesus and because of the word of God…"
Here, again, is a portrait of the saints. Note that "we are given authority to judge the world." How so? Through our godly behavior — just as righteous Noah judged the pre-Flood age during his time:
1 Corinthians 6:2a: "Or do you not know that the saints will judge the world?"
As we obey the Gospel, we are setting the standard by which all others are to live.
Suppose we paraphrase the follow-up passage in Revelation 20 to assist us further. This describes the contrast between the godly and the godless (dead outside of Christ):
Revelation 20:6 paraphrased: “Blessed and holy is the one who has a part in the first resurrection [baptism into Christ]; over these [Christians], the second death [spiritual death] has no power, but they [Christians] will be priests of God and of Christ [all Christians are priests and royalty: 1 Peter 2:9] and will reign with Him for a thousand years [until Christ’s return].”
The “thousand years” in this passage is metaphorical: it is an indeterminate amount of time, symbolizing that which remains until Christ brings the world to an end.
Also, note that as the faithful, we are “priests of God and of Christ,” and we are “reigning with Him” now – just as this passage from Revelation tells us. Hear Peter’s First Letter to the saints:
1 Peter 2:9: “But you [Christians] are A CHOSEN RACE, A royal PRIESTHOOD, A HOLY NATION, A PEOPLE FOR God’s OWN POSSESSION...”
In Christ, we are the faithful seated on the thrones. And, as with Revelation 20:4, we are also those who are "[persecuted] in the name of Jesus."
The “souls under the altar” (Rev. 6:9) represent the totality of saints — including us — that have suffered for the Word of God. Our destiny is eternal life.