What does Revelation 6:9 mean?
In this verse we learn that, when the fifth seal was opened, John saw the souls of martyrs under the altar. The most common interpretation of this passage holds that these are the martyrs who believe on Christ as Savior during the first half of the tribulation. These, then, would be people who come to faith in Christ after the rapture of the church. These souls are likely connected to, or the same as, the believers "sealed" in Revelation chapter 7.John saw the martyrs' souls "under the altar" in heaven. Their bodies awaited resurrection at the return of Jesus to the earth (Daniel 12:1–2). The martyrs' place under the altar indicates that they had sacrificed their lives as an act of worship to God. Exodus 29:12 instructed the Old Testament priests to pour out most of the blood of a sacrificial bull at the base of the altar of sacrifice, so it is not surprising to find the martyrs under the altar.
These believers were martyred because they held fast to the Scriptures and shared the gospel with others. Marshaled under Satan, hostile unbelievers have always aggressively opposed believers. History includes many accounts of evil cultures taking the lives of God's people. Such hatred of believers will reach a boiling point in the tribulation.