Of the fall of the devil and the angels.
And so we are clearly shown that out of that number of them some of the leaders fell, by the lamentations of Ezekiel and Isaiah, in which we know that the prince of Tyre or that Lucifer who rose in the morning is lamented with a doleful plaint: and of him the Lord speaks as follows to Ezekiel: “Son of man, take up a lamentation over the prince of Tyre, and say to him: Thus says the Lord God: You were the seal of resemblance, full of wisdom, perfect in beauty. You were in the pleasures of the paradise of God: every precious stone was your covering: the sardius, the topaz and the jasper, the chrysolyte and the onyx and the beryl, the sapphire and the carbuncle and the emerald: gold the work of your beauty, and your pipes were prepared in the day that you were created. You were a cherub stretched out and protecting, and I set you in the holy mountain of God, you have walked in the midst of the stones of fire. You were perfect in your ways from the day of your creation, until iniquity was found in you. By the multitude of your merchandise your inner parts were filled with iniquity and you have sinned; and I cast you out from the mountain of God, and destroyed you, O covering cherub, out of the midst of the stones of fire. And your heart was lifted up with your beauty: you have lost your wisdom in your beauty, I have cast you to the ground: I have set you before the face of kings, that they might behold you. You have defiled your sanctuaries by the multitude of your iniquities and by the iniquity of your traffic.” Isaiah also says of another: “How are you fallen from heaven, O Lucifer, who rose in the morning? How are you fallen to the ground, that wounded the nations? And you said in your heart, I will ascend into heaven, I will exalt my throne above the stars of God, I will sit in the mountain of the covenant, in the sides of the north. I will ascend above the heights of the clouds. I will be like the Most High.” But Holy Scripture relates that these fell not alone from that summit of their station in bliss, as it tells us that the dragon dragged down together with himself the third part of the stars. One of the Apostles too says still more plainly: “But the angels who kept not their first estate, but left their own dwelling, He has reserved in everlasting chains under darkness to the judgment of the great day.” This too which is said to us: “But you shall die like men and fall like one of the princes,” what does it imply but that many princes have fallen? And by these testimonies we can gather the reason for this diversity; viz., either that they still retain those differences of rank (which adverse powers are said to possess, after the manner of holy and heavenly virtues) from the station of their former rank in which they were severally created, or else that, though themselves cast down from heavenly places, yet, as a reward for that wickedness of theirs in which they have graduated in evil, they claim in perversity these grades and titles of rank among themselves, by way of copying those virtues which have stood firm there.
Source: Conferences (New Advent)