Revelation 6:9-10

When 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]. They 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?” [Gen 4:10; Ps 79:10; 94:3; Zech 1:12]