Revelation 6:9-11

9When He (the Lamb) broke open the fifth seal, I saw underneath the altar the souls of those who had been slaughtered because of the word of God, and because of the testimony which they had maintained [out of loyalty to Christ].

10They cried in a loud voice, saying, “O Lord, holy and true, how long now before You will sit in judgment and avenge our blood on those [unregenerate ones] who dwell on the earth?”

11Then they were each given a white robe; and they were told to rest and wait quietly for a little while longer, until the number of their fellow servants and their brothers and sisters who were to be killed even as they had been, would be completed.