St. Michael was one of the most popular saints in medieval Europe, in both Catholicism and Orthodoxy. This is rather odd, because Michael is not a saint at all, but an angel. Saints were human once, and performed some noted service to Christianity; the most prestigious ones were martyred for their faith. Saints are in heaven with God, and in the Middle Ages acquired the function of intercession: you could pray to them, and they would be deputized with answering; they might specialize in providing a particular type of miracle, and amass a particular set of devotees.
Angels are different. In both the Old and New Testaments, angels function as messengers of God. Gabriel, Uriel, and Michael are the three best-known. They had never enjoyed a human existence, but were always semi-divine members of the court of heaven. As such, one would think that they would enjoy a Christian cult like that of the most powerful saints, but only Michael seems to have. (One would think that Old Testament prophets like Moses, Elijah, or Isaiah could be Christianized in this way as well, but one generally does not find churches dedicated to them, prayers addressed to them, or accounts of their lives included in saints legendaries.)
Why St. Michael should have enjoyed church and guild dedications, heard Christian prayers, had his own feast day (Michaelmas, September 29), been included in the Golden Legend, etc., is a mystery I’d like to explore more. Gabriel, despite his appearance to the Virgin Mary herself, was nowhere near as popular. The only thing I can think of right now is artistic: St. Michael was often shown battling the devil, as he does in Revelation. I’m convinced that one of the main reasons why St. George was so popular was simply because he was shown fighting the dragon; people loved the action. In England, George and Michael were sometimes paired, each one overcoming his scaly enemy.
This leads to a very important aspect of St. Michael’s patronage: he was a warrior saint. He protected and encouraged “those who fight,” as they fought. This was not entirely a Christian thing to do, but once the Church endorsed crusading (Holy War to liberate Jerusalem from the infidel), it was only natural that different saints should be accepted as specialists in warfare – whether practiced on crusade or not. Once the English managed to monopolize St. George in the context of the Hundred Years’ War (following the Battle of Crécy in 1346), the French turned increasingly to St. Michael. Colette Beaune talks about this in her Naissance de la nation France; the Norman monastery of Mont-St-Michel played a role, as did the foundation of the French Order of St. Michael in 1469.
Depictions of St. Michael followed suit. Normally, he was shown as an angel, dressed in dalmatic. As the Middle Ages wore on, however, he acquired more and more pieces of military equipment, such as helmet, breastplate, greaves, and shield. And on the shield – a coat of arms.
What coat of arms does St. Michael bear? When he’s dressed as a knight, he has the chance to display arms on his shield, and perhaps also on his breastplate. Sometimes he holds a banner, although this does not necessarily display his personal arms.
Unfortunately, no clear patterns have emerged. Here are some rough groupings:
1. “Michael” means “Who is like God?” with the implied answer, “No one.” Thus, St. Michael can bear a shield bearing a Latin form of his name as a motto: Quis ut Deus?
2. As a Christian warrior, it is only natural for Michael to bear a cruciform shield.
2a. Oftentimes, this shield is simply St. George’s red cross on white.
2b. Red and gold are also seen.
2c. Also gold on gold!
2d. Sometimes charges appear in the quarters.
2e. A Russian Michael bears a Russian cross:
3. Several shields aren’t heraldic at all, but are simply blank and/or decorative and/or functional.
4. Many shields feature rays of light or flames.
5. A couple of shields bear an escarbuncle, which is a charge derived from the way a shield is constructed.
6. St. Michael occasionally bears a shield of one of his votaries – in this case, the king of France.
7. I found one that seems to be covered in feathers, like Michael’s wings.
So as you can see, unlike with St. George, who is almost universally identified with Argent, a cross Gules, artists of St. Michael could get quite creative with his coat of arms. If this has any meaning, I assume that it’s because St. Michael is even more heavenly than your average warrior saint, and not constrained by any heraldic rules.