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.
So too Gabriel, who is called God’s strength, was sent to Mary. He came to announce the One who appeared as a humble man to quell the cosmic powers. Thus God’s strength announced the coming of the Lord of the heavenly powers, mighty in battle. Raphael means, as I have said, God’s remedy, for when he touched Tobit’s eyes in order to cure him, he banished the darkness of his blindness. Thus, since he is to heal, he is rightly called God’s remedy.
(from todays Office of Readings)