As the Apostle says: “Are they not all ministering spirits sent forth to minister for those who will inherit salvation?” (Heb 1:4). The very name Michael is translated as “help of God,” and accordingly this work is assigned particularly to him.
Daniel also said that in the days of the final trials Michael will come to the aid of the Church: “At that time Michael shall arise, the great prince who stands for the sons of thy people. There shall be a time of trouble, such as never has been since there were men until that time. But at that time all of thy people who are found written in the book shall be saved” (Dan 12:1).
The angels are called his angels according to a certain manner of speech which we find elsewhere: “For their angels in heaven always see the face of My Father Who is in heaven” (Mt 18:10). “Their” means those who through faith have begun to be citizens in Christ. So here also they are “his angels,” because they are understood to be under the protection of one Head and endowed with one Life-giving Spirit. Caesarius plainly identifies Michael with Christ: “Understand Michael as Christ, and his angels as the saints” (PL 25, 2334).
The devil and his angels should be understood not only as those like him in nature and will, but also as men caught in their snares and made such companions of the devil. Because of the properties of the will, it is said of the devil, “An enemy man has done this” (Mt 13:28), and of Judas, “One of you is a devil” (Jn 6:70).
The devil is certainly so called because of the twofold composition of his army. Having been defeated, as it says, he was torn away by those who, after rejecting him and receiving faith in Christ, no longer repeated his errors. In them there is unshaken love “from a pure heart, a good conscience, and sincere faith” (1 Tim 1:5).
Or, since the Church has already been separated from every admixture of evil and glorified by future blessedness, no place remains for the devil and his angels to seduce the evil and tempt the good. Therefore the Psalm, indicating the devil, says: “I passed by, and behold, he was not; I sought him, and his place was not found” (cf. Ps 36:36).