Alright, I have read everyone's posts and the thing that is missing is a Catholic understanding of Saint Michael, the Archangel, and as I used to be a Catholic Sunday School teacher (before coming home to paganism) and was very closely associated with Saint Michael, I would like to contribute to this.
According to Christian Tradition:
Saint Michael, along with the other eight archangels (Lucifer, Gabriel, Uriel, Raphael, Baruchiel, Jehudiel, and Sealtiel) were created prior to the creation of mankind. The archangels are one of nine chiors (spheres) of angels. The word "archangel" means "chief/first angel/messenger".
Saint Michael is the patron of exorcists, priests, soldiers, warriors, and generally anything that protects other people. It is told that at the end of days Michael with battle with "the great serpent" (a manifestation of evil often thought to be The Satan) and he will bind and shackle him and cast him into oblivion.
Part of the reason that Michael was so revered in Celtic lands is because of his ability to protect from evil and "keep out the devil" (which the Christians assume is everything that isn't subjugate to their god). Also, in Ireland, Saint Patrick was a devotee to Michael and thus Michael's name got plastered around on that island as well.
Just a quick note on some of the things I read in other people's responses:
@Skydove: you might want to check the validity of the book you are reading because the bible states specifically that the angels that guarded the gates of Eden after the expulsion were Cherubim (another sphere of angels). Further, if the "burning bush experience" is attributed to any angel in Christianity it is Metatron, the voice of YHWH.
Okay, that's it for this young bard's Christianity lesson....