John Wesley on Revelation 6:9

And when he opened the fifth seal - As the four former seals, so the three latter, have a close connexion with each other. These all refer to the invisible world; the fifth, to the happy dead, particularly the martyrs; the sixth, to the unhappy; the seventh, to the angels, especially those to whom the trumpets are given. And I saw - Not only the church warring under Christ, and the world warring under Satan; but also the invisible hosts, both of heaven and hell, are described in this book. And it not only describes the actions of both these armies upon earth; but their respective removals from earth, into a more happy or more miserable state, succeeding each other at several times, distinguished by various degrees, celebrated by various thanksgivings; and also the gradual increase of expectation and triumph in heaven, and of terror and misery in hell. Under the altar - That is, at the foot of it. Two altars are mentioned in the Revelation, "the golden altar" of incense, Rev 9:13; and the altar of burnt - offerings, mentioned here, and Rev 8:5, 14:18, 16:7. At this the souls of the martyrs now prostrate themselves. By and by their blood shall be avenged upon Babylon; but not yet, whence it appears that the plagues in the fourth seal do not concern Rome in particular.