@Luco42 IMO it has to do with the crufixes on their chests. so the sound wave is resonating with the type of metal inside their crufixes and intensifies the wave locally
I wonder if priest Not-Xellos is also a time traveller, seeing as he "granted" miracles, and according to Makito it's magic for people of this era (the "Any sufficiently advanced technology is indistinguishable from magic" thing).
I find it funny how the priest aren't being condemned when their "miracles" are more witch like than the ones they're accusing of being a witch. Religeon is funny like that.
Building on some ideas @donoghu mentioned, here's a theory:
The knives have magnetized tips and fly with a very powerful electromagnet positioned behind the target. Then arena, for instance, could have an electromagnet behind each wall allowing the knives to be aimed anywhere within the arena, as well as potentially changing the path midflight. String is used to make the knives initially float, but once the electromagnet is turned on, the knives move fast enough to break the string and leave them clean for examination.
Makito's epiphany from the stigmata was that the devil's marks they've been making were made from sheet copper (ch 10), which is a common material in modern electromagnets. Liese's stigmata may have been caused from some sort of copper exposure or allergy (?).
@Ryrie
that's the irony of superstitious Christianity who drank the kool-aid performing feats with no known/obvious cause is clearly the work of heresy and devilry, but look here I wear a cross thus my amazing feat is actually a miracle of god
"the secret power of appearing to make impossible things happen by saying special words or doing special things"
Witches: May or may not using "magic"
Iron Hammer Bishops: Make people awe using things people find impossible and special only to them
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Inquisitors: Should I hang them all?