A Sermon of Pope St. Gregory the Great
We speak of nine choirs of Angels, because we know, by the testimony of Holy Scripture, that there are the following: Angles, Archangels, Virtues, Powers, Principalities, Dominations, Thrones, Cherubim, and Seraphim. Nearly every page of Scripture witnesses to the fact that there are Angels and Archangels. The prophetic books, as has been noted often, speak of Cherubim and Seraphim. Four more orders are enumerated by Paul the Apostle, writing to the Ephesians, when he says, "Above every Principality and Power and Virtue and Domination." And again, writing to the Colossians, he says, "Whether Thrones, or Powers, or Principalities, or Dominations." When, then, we add the Thrones to those he mentions in Ephesians, there are five orders, to which are to be added Angels, Archangels, Cherubim and Seraphim, certainly making nine orders of Angels in all.
It must be realized that "Angel" is the name of their office, not of their nature. For the holy spirits of the heavenly homeland are always spirits, but they cannot always be called Angels; they are Angels only when they are announcing something. And so the Psalmist says, "He who makes spirits His Angels," as if he said plainly that, when He wills, He uses as Messengers those spirits who are always His. Those who announce less important things are called Angels, and those who announce the highest things are called Archangels. And so not any Angel but the Archangel Gabriel was sent to Mary; for this ministry it was fitting to have the highest Angel, since he was to announce the greatest news of all. These Archangels are also given special names to describe their particular virtue. For Michael means "Who is like unto God?" Gabriel means "Strength of God," and Raphael "Medicine of God."
Whenever something is to be done needing great power, Michael is sent forth so that from his action and from his name we can understand that no one can do what God can do. Hence that old enemy who through pride desired to be like God, saying, "I will scale the heavens, I will set up my throne, I will be like the Most High," is shown at the end of the world left to his own strength, and about to undergo the final punishment, as destined to fight with Michael the Archangel. Similarly, Gabriel was sent to Mary; he who is called "strength of God" came to announce Him who deigned to appear in humility to conquer the powers of the air. And Raphael is interpreted, as we said, "Medicine of God," for when he touched the eyes of Tobias to do the work of healing, he dispelled the night of his blindness.