From the Office of Readings for the Feast of the Archangels:
Now a great sign appeared in heaven: a woman, adorned with the sun, standing on the moon, and with the twelve stars on her head for a crown. She was pregnant, and in labour, crying aloud in the pangs of childbirth. Then a second sign appeared in the sky, a huge red dragon which had seven heads and ten horns, and each of the seven heads crowned with a coronet. Its tail dragged a third of the stars from the sky and dropped them to the earth, and the dragon stopped in front of the woman as she was having the child, so that he could eat it as soon as it was born from its mother. The woman brought a male child into the world, the son who was to rule all the nations with an iron sceptre, and the child was taken straight up to God and to his throne, while the woman escaped into the desert, where God had made a place of safety ready, for her to be looked after in the twelve hundred and sixty days.
And now war broke out in heaven, when Michael with his angels attacked the dragon. The dragon fought back with his angels, but they were defeated and driven out of heaven. The great dragon, the primeval serpent, known as the devil or Satan, who had deceived all the world, was hurled down to the earth and his angels were hurled down with him. Then I heard a voice shout from heaven, ‘Victory and power and empire for ever have been won by our God, and all authority for his Christ, now that the persecutor, who accused our brothers day and night before our God, has been brought down. They have triumphed over him by the blood of the Lamb and by the witness of their martyrdom, because even in the face of death they would not cling to life. Let the heavens rejoice and all who live there; but for you, earth and sea, trouble is coming – because the devil has gone down to you in a rage, knowing that his days are numbered.’
As soon as the devil found himself thrown down to the earth, he sprang in pursuit of the woman, the mother of the male child, but she was given a huge pair of eagle’s wings to fly away from the serpent into the desert, to the place where she was to be looked after for a year and twice a year and half a year So the serpent vomited water from his mouth, like a river, after the woman, to sweep her away in the current, but the earth came to her rescue; it opened its mouth and swallowed the river thrown up by the dragon’s jaws. Then the dragon was enraged with the woman and went away to make war on the rest of her children, that is, all who obey God’s commandments and bear witness for Jesus. (Revelation 12:1-17)
From a sermon of Pope St Gregory the Great:
The word “angel” denotes a function rather than a nature
You should be aware that the word “angel” denotes a function rather than a nature. Those holy spirits of heaven have indeed always been spirits. They can only be called angels when they deliver some message. Moreover, those who deliver messages of lesser importance are called angels; and those who proclaim messages of supreme importance are called archangels. And so it was that not merely an angel but the archangel Gabriel was sent to the Virgin Mary. It was only fitting that the highest angel should come to announce the greatest of all messages.
Some angels are given proper names to denote the service they are empowered to perform. In that holy city, where perfect knowledge flows from the vision of almighty God, those who have no names may easily be known. But personal names are assigned to some, not because they could not be known without them, but rather to denote their ministry when they came among us. Thus, Michael means “Who is like God”; Gabriel is “The Strength of God”; and Raphael is “God’s Remedy.”
Whenever some act of wondrous power must be performed, Michael is sent, so that his action and his name may make it clear that no one can do what God does by his superior power. So also our ancient foe desired in his pride to be like God, saying: I will ascend into heaven; I will exalt my throne above the stars of heaven; I will be like the Most High. He will be allowed to remain in power until the end of the world when he will be destroyed in the final punishment. Then, he will fight with the archangel Michael, as we are told by John: A battle was fought with Michael the archangel.