(Based on his 1911 Catholic Encyclopedia entry)
St. Michael is one of the principal angels. His name (translated from Hebrew, “Who is like God?“) is the war-cry of the good angels in the battle fought in heaven against the devil and his followers. His name is recorded four times in Scripture:
Daniel 10 — Gabriel says to Daniel, when he asks God to permit the Jews to return to Jerusalem: “The Angel of the kingdom of the Persians resisted me … and, behold Michael, one of the chief princes, came to help me … and none is my helper in all these things, but Michael your prince.”
Daniel 12 — An angel speaking of the end of the world and the Antichrist says: “At that time shall Michael rise up, the great prince, who stands for the children of your people.”
Jude 1 — St. Jude alludes to an ancient Jewish tradition of a dispute between Michael and Satan over the body of Moses: “[The archangel Michael] did not venture to pronounce a reviling judgment upon [Satan] but said, ‘May the Lord rebuke you!’”
Revelation 12 — St. John speaks of the great conflict at the end of time, which reflects also the battle in heaven at the beginning of time: “Then war broke out in heaven; Michael and his angels battled against the dragon. The dragon and its angels fought back, but they did not prevail and there was no longer any place for them in heaven. The huge dragon, the ancient serpent, who is called the Devil and Satan, who deceived the whole world, was thrown down to earth, and its angels were thrown down with it.”
Following these Scriptural passages, Christian tradition gives to St. Michael four offices:
(1) To fight against Satan.
(2) To be the champion of God’s people, the Jews in the Old Law, the Christians in the New Testament; therefore he was the patron of the Church, and of the orders of knights during the Middle Ages.
(3) To rescue the souls of the faithful from the power of the enemy, especially at the hour of death.
(4) To call men’s souls away from earth and bring their souls to judgment.
Regarding his rank in the celestial hierarchy opinions vary; St. Basil and other Greek Fathers place St. Michael over all the angels; they say he is called “archangel” because he is the prince of the other angels; others believe that he is the prince of the seraphim, the first of the nine angelic orders. But, according to St. Thomas Aquinas he is the prince of the last and lowest choir, the angels.