Inanimate objects (like an altar built of bronze) cannot speak or think. Clearly, we have here a case of metonymy, where the thing stands for what it represents. Sinners used the altar to confess their identity (sinful) and conduct (sins) and seek God’s forgiveness through a substitute (which foreshadowed the coming sacrifice of Jesus Christ, the Lamb of God, John 1:29). It is these who receive atonement through the altar who agree with the angel’s song. Furthermore, so many of these forgiven sinners had been martyred over the years and they’re pictured as being “under the altar” (Revelation 6:9), crying out for God’s vengeance on their blood. Revelation 12:17, Revelation 13:7, and Revelation 13:15 make clear that those who heard the gospel (Revelation 14:6) yet “shed the blood of saints and prophets” (Revelation 16:6). “The altar” (= also saints under the altar) agrees that their murderers have received what they deserve.
7 And I heard the altar saying, “Yes, Lord God the Almighty, true and just are your judgments!”