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. 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 action and name may make it clear that no one can do what God does by his own superior power.

—Homily 34 on the Gospels 8-9