John sees the souls of them that were slain for the word of God, and for the testimony which they held. They seek to be avenged for their martyrdom and are told to wait until their fellowservants also and their brethren, that should be killed as they were, should be fulfilled.
And when he had opened the fifth seal, I saw under the altar the souls of them that were 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, dost thou not judge and avenge our blood on them that dwell on the earth? And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season, until their fellowservants also and their brethren, that should be killed as they [were], should be fulfilled. (Revelation 6:9-11)