REVELATION CHAPTER SIX VS.9 - 11
Rev. 6:9 - 11
And when he opened the fifth seal, I saw underneath the altar the souls of them that had been slain for the word of God, and for the testimony which they held; and they cried with a great voice, saying, How long. O Master, the holy and true, dost thou not judge and avenge our blood on them that dwell on the earth? And there was given them to each one a white robe; and it was said unto them, that they should rest yet for a little time, until their fellow-servants also and their brethren, who should be killed even as they were, should have fulfilled their course.
These martyrs are people who came to faith since the resurrection of the saints possibly by the preaching of the 144,000 Jewish Evangelists (Rev 7:1 - 8) When the saints disappeared possibly this caused many people who had heard of ‘the resurrection’ (or 'rapture') to finally make a decision to repent of their sins and believe in Jesus Christ for their salvation.