"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. 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?'" (Revelation 6:9-10).
To understand this scripture, we must remember the context. John was witnessing a vision while he was "in the Spirit" (Revelation 4:2). Under inspiration he was seeing future events depicted in symbolism. The fifth seal is figurative of the Great Tribulation, a time of world turmoil preceding Christ's return.
In this vision, John sees under the altar the martyred believers who sacrificed their lives for their faith in God. These souls figuratively cry out, "Avenge our blood!" This can be compared to Abel's blood metaphorically crying out to God from the ground (Genesis 4:10). Though neither dead souls nor blood can actually speak, these phrases figuratively demonstrate that a God of justice will not forget the evil deeds of mankind perpetrated against His righteous followers.
This verse does not describe living souls that have gone to heaven. The Bible confirms that "no one has ascended to heaven but He who came down from heaven, that is, the Son of Man who is in heaven [Jesus Christ]" (John 3:13). Even righteous King David, a man after God's own heart (Acts 13:22), was described by Peter as being "dead and buried" (Acts 2:29), not alive in heaven or some other state or location (verse 34).
The description of souls "under the altar" is obviously symbolic. In 1 Thessalonians 4:16 Paul makes it clear that deceased Christians remain in the grave until Christ returns. When we allow the Bible to interpret itself, Revelation 6:9 cannot mean that anyone is in heaven now. Rather, it is both a prophecy that martyrdom would occur and that it would continue until the second coming of Christ.
You can read more about how this section of Scripture fits into the overall framework of Bible prophecy in our booklet The Book of Revelation Unveiled under the section about "the fifth seal."