September 29 was formerly dedicated to all the angels (Introit, Collect, Gradual, Communion), wherefore Pope Boniface II, about A.D. 530, chose that date to dedicate a church in the great circus, at Rome, to St. Michael. The Mass composed for the occasion has since been appointed for the 18th Sunday after Pentecost; it still relates to the dedication of a church. The present Mass was composed more recently.
The Hebrew meaning of Michael is: Who is like God, and recalls the combat which took place in heaven between "the archangel of God who deserved to be placed at the head of the heavenly host" and the devil. As we have fallen through sin into the power of Satan, it is St. Michael's part to continue the fight for our deliverance (Alleluia, and Prayer after Mass): wherefore our guardian angels are subordinate to him. St. Michael conquers Satan's pride and obtains humility for us. It is also he who presides over the worship of adoration rendered to the most High, for he offers to God the prayers of the saints symbolized by incense whose smoke rises towards heaven (Offertory, Blessing of the incense). When a Christian has left this world, we pray that the standard-bearer St. Michael should introduce him into heaven; he is also often represented with the scales of divine justice wherein souls are weighed. His name is mentioned in the Confiteor, after that of Mary who is the Queen of Angels.
St. Michael was the protecting angel of the synagogue, as he is now of the Church which has succeeded it. To him the liturgy attributes the revelation of the future made to St. John in the Apocalypse (Epistle).
Bless the Lord all ye His angels: you that are mighty in strength, and execute His word, hearkening to the voice of His orders. * Bless the Lord, O my soul: and let all that is within me bless His holy name.
(Psalm 102:20,1 from the introit of Mass)
Deus, qui miro ordine, Angelorum ministeria hominumque dispensas: concede propitius; ut, a quibus tibi ministrantibus in caelo semper assistitur, ab his in terra vita nostra muniatur.
O God, who in a wonderful I order hast established the ministry of angels and of men, mercifully grant that even as Thy holy angels ever do Thee service in heaven, so at all times they may defend us on earth.
Lesson from the Apocalypse of Blessed John the Apostle.
In those days : God signified the things which must shortly come to pass, sending by His angel to His servant John, who hath given testimony to the word of God, and the testimony of Jesus Christ, what things soever he hath seen. Blessed is he that readeth and heareth the words of this prophecy, and keepeth those things which are written in it; for the time is at hand. John to the seven churches which are in Asia: Grace be unto you and peace from Him that is, and that was, and that is to come; and from the seven spirits which are before His throne; and from Jesus Christ, who is the faithful witness, the first-begotten of the dead, and the prince of the kings of the earth, who hath loved us, and washed us from our sins in His own blood.
Dum sacrum mysterium cerneret Joannes, Archangelus Michael tuba cecinit: Ignosce, Domine Deus noster, qui operis librum, et solvis signacula ejus, alleluia.
While John was beholding the sacred mystery, the Archangel Michael sounded a trumpet. Forgive us, O Lord our God, Thou who openest the book, and loosest the seals thereof, alleluia.
(Antiphon at the Magnificat)