Revelation 6:9-10

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?”