From the Treasury of Daily Prayer:

The name of the archangel St. Michael means “Who is like God?” Michael is mentioned in the Book of Daniel (12:), as well as in Jude (v. 9) and Revelation (12:7). Daniel portrays Michael as the angelic helper of Israel who leads the battle against the forces of evil. In Revelation, Michael and his angels fight against and defeat Satan and the evil angels, driving them from heaven. Their victory is made possible by Christ’s own victory over Satan in His death and resurrection, a victory announced by the voice in heaven: “Now the salvation and the power and the kingdom of our God and the authority of His Christ have come” (Revelation 12:10). Michael is often associated with Gabriel and Raphael, the other chief angels or archangels who surround the throne of God. Tradition names Michael as the patron and protector of the Church, especially as the protector of Christians at the hour of death.