These names refer to categories of angels, those pure spirits who adore God and serve as His messengers. Since the fourth century, nine choirs or types of angels were identified in the Bible and popularized in the Middle Ages by various theologians and writers, like St. Thomas Aquinas, Dante, Hildegard of Bingen and John Scotus Erigena. Together, they form the “hosts of heaven," i.e., God’s army of angels. Almighty God is called “Lord of Hosts" (in Hebrew, “Yahweh Sabaoth") over three hundred times in the Old Testament (particularly in the books of the prophets) and twice in the New Testament (Rom 9:29 and Jas 5:4).
For example, in Isaiah 6:1-2, the prophet has a vision of the heavenly liturgy: “In the year that King Uzziah died, I saw the Lord sitting upon a throne, high and lifted up; and His train filled the temple. Above Him stood the seraphim; each had six wings: With two He covered His face, and with two He covered his feet, and with two He flew. And one called to another and said: “Holy, holy, holy (is the) Lord of hosts. The whole earth is full of His glory." A similar account is found in the Book of Revelation, which likewise described how the angels surround the throne of God and sing praises (cf. Rev 5:1, 7:11). For good reason, at Mass, we pray in the Sanctus: “Holy, holy, holy, Lord God of host …." or in Latin, “Sanctus, sanctus, sanctus, Dominus Deus Sabaoth …"
The nine choirs of angels also are categorized into three groups: The first three choirs see and adore God directly. As mentioned in the citation from Isaiah, the seraphim, meaning “the burning or fiery ones," have the most intense “flaming" love for God and comprehend Him with the greatest clarity. In Isaiah 6:6, a seraphim took a burning ember from the altar and touched the lips of Isaiah, purging him of sin so he could prophesy in the name of the Lord. (Interestingly, Lucifer, which means “light bearer," was one of the seraphim whose beautiful light was changed into darkness because of his sinfulness.)
The second choir is the cherubim, which means “fullness of wisdom." They contemplate God’s divine providence and plan for His creatures. The prophet Ezekiel described them as follows: “Their form was human, but each had four faces and four wings, and their legs went straight down; the soles of their feet were round. They sparkled with a gleam like burnished bronze. … Each of the four had the face of a man, but on the right side was the face of a lion, and on the left side the face of an ox, and finally each had the face of an eagle. Their faces and their wings looked out on all their four sides; they did not turn when they moved, but each went straight forward. … Human hands were under their wings, and the wings of one touched those of another. Each had two wings spread out above so the they touched one another’s, while the other two wings of each covered his body. … They seemed like torches, moving to and fro. … The fire gleamed, and from it came forth flashes of lightning" (Ez 1:4-14). Remember that when God expelled Adam and Eve from the Garden of Eden, He “stationed the cherubim and the fiery revolving sword, to guard the way to the tree of life" (Gen 3:34). To signify the presence of God in the holy of holies of the temple (or earlier, the tent dwelling), the veil covering the entrance was woven of blue, purple and scarlet yarns embroidered with cherubim, and atop the Ark of the Covenant was the propitiatory (the mercy seat, or throne of God) that had two gold cherubim at each end with their wings extended over it.
Lastly, the thrones, symbolizing divine justice and judicial power, contemplate God’s power and justice. Reference to “thrones" is found in Colossians 1:16, and in Jewish apocryphal literature, the Book of Enoch and the Testament of Levi.
The next three choirs fulfill God’s providential plan for the universe: The dominations or dominions, whose name evokes authority, govern the lesser choirs of angels. The virtues, whose name originally suggested power or strength, implement the orders from the dominations and govern the heavenly bodies. Lastly, the powers confront and fight against any evil forces opposed to God’s providential plan. (Again reference is made to these choirs in Colossians 1:16.)
The last three choirs are involved directly in human affairs: The principalities care for earthly principalities, such as nations or cities. The archangels deliver God’s most important messages to mankind, and Scripture identifies three by name — Sts. Michael, Raphael and Gabriel. St. Michael, whose name means, “one who is like God," led the army of angels who cast Satan and the rebellious angels into hell; at the end of time, he will wield the sword of justice to separate the righteous from the evil (cf. Rev 12:7-9). St. Gabriel, whose name means “strength of God," announced to Mary that she had been chosen as the mother of the Savior (cf. Lk 1:26-38). Finally, St. Raphael, whose name means “remedy of God," cured the blind man Tobit (cf. Tb 5).
Finally, we have the angels, who appear throughout the Old and the New Testament, leading and guarding the people, like during the Exodus or freeing St. Peter from prison. Here we must remember our guardian angels. Jesus said, “See that you never despise one of these little ones. I assure you, their angels in heaven constantly behold my heavenly Father’s face" (Mt 18:10), indicating that each of us has a guardian angel. The catechism states, “From infancy to death, human life is surrounded by their watchful care and intercession" (No. 336). Most of us at an early age learned the little prayer to our guardian angel: “Angel of God, my guardian dear, to whom God’s love commits me here. Ever this day be at my side, to light, to guard, to rule and to guide." Some of the saints were blessed to see their guardian angel, like St. Pio (Padre Pio), St. Frances of Rome and St. Elizabeth of Hungary.
Therefore, may we always be mindful of these heavenly protectors. Let us pray as at the Mass in their honor: “O God, who in your unfathomable providence are pleased to send your holy angels to guard us, hear our supplication as we cry to you, that we may always be defended by their protection and rejoice eternally in their company."