Faith Magic Theory
Faith Magic I think might not actually be magic in & of itself, just a fundamental force in the world that other creatures such as the undead react to (Red & Black vamps combusting on contact for example), when normally faith is just faith, doesn’t do anything really magically speaking. It explains why Michael was able to call up fire in Grave Peril; Vampire proximity. You’ll notice also that their swords don’t do anything unless there’s a baddy nearby, which reinforces the theory that faith magic is simply the way other magical creatures react to faith, hence why mortal Susan’s cross glowed against Mavra. There’s no actual power, just thoughts, maybe a bit of energy at the most. Think about the way the swords work; it has been stated numerous times that it’s simply “Michael’s faith multiplied a thousandfold”. That makes sense with the theory, because the swords themselves don’t actually DO much of anything other than:
& B) exude an aura that only demons n nasties are harmed by, & that only wizards sense (since they can sense normal faith).
They never light non-monster things on fire, they never glow without the presence of adversaries, they don’t even really cut more than the average sword. It’s just that faith, which monsters normally react to in varying degrees, multiplied so much that even creatures which are only minorly affected by faith (Faeries, Denarians, Hobs) are now receiving enough of it to be damaged.
There’s also the fact that you don’t really see cases in the Dresdenverse of magic being “multiplied”. Added to, gathered, coalesced, focused, directed, but not amplified by some sort of magical tool. Harry references physics many times; you can’t create something from nothing. It is said multiple times that the swords actually multiply or amplify faith, so that might mean that it really is just a sensation or thought that is given depth by the swords, depth enough to have an effect on monsters that normally wouldn’t even blink at a cross in the face.
What does everyone think?