At the opening of the sixth seal....a great earthquake, &c. Many think that these dreadful signs, of the sun turning black, &c. are not to happen till the time of antichrist, a little before the end of the world. See Matthew xiv.; Luke xxi.; Isaias xiii. and xxxiv.; Ezechiel xxxiii.; Daniel xii.; &c. Others apply these prodigies to God’s visible chastisements, on the heathen emperors and persecutors of the Christians, before the first Christian emperor Constantine. (Witham)

And I saw. The sixth seal being opened, St. John sees painted before him the severe and terrible manner in which the Almighty would revenge himself on his enemies. It may refer either to the time of Constantine, when we behold the Christian religion triumphing on the ruins of paganism, and after his death, and that of his sons, the empire of Rome given up a prey to barbarians, Rome itself taken and pillaged, and all the provinces thrown into dreadful disorder and consternation; or it may likewise refer to the day of general judgment, when the Almighty will make sinners drink the wine of his indignation, in presence of all the just; of which dreadful time of vengeance all other particular judgments are only imperfect figures. (Victorin; Ven. Bede; Tichon)

St. John, in imitation of the ancient prophets, makes use of the earthquake, &c. hyperbolically, to mark more strongly the dreadful and horrible evils with which the Roman empire, and its persecuting emperors, were to be overwhelmed. Rome itself was filled with wars and seditions, both at home and abroad. The emperors were all destroyed by the Almighty in a most singular manner; witness Maxentius, who was thrown headlong into the Tiber; Maximin Jovius, who, under a horrible and incurable disease, owned the hand of the Almighty afflicting him; Maximin Daia, who, being overcome in battle, fled away in disguise, and at last, seized with a strange disease, his bowels were all consumed, he lost his eyes, and died reduced to a mere skeleton. Witness likewise Licinius, who, being engaged with Constantine, was always beaten, and at length strangled. Maximian also, the rival of Constantine, who strangled himself in Marseilles, where he had been confined. (Calmet)