I'm only a few pages in, but this feels forced, the spirit's invisible, but they are able to interact with the world no? Just do something flashy?
I'll edit this accordingly once I reach the end of the chapter I suppose.
EDIT: So the fire spirit already made the suggestion to "show off" but the suggestion was rejected for some reason or another. On top of that, the country does know of spirits, maybe it's high-level knowledge so the average guild wouldn't know about it, thus they don't believe MC but ugh.
At least the shitstorm that hit in the end was fun, I'll stick around and see where this goes.
EDIT 2: Nevermind, they touch on this in a later chapter, just showing off won't do, I assumed he was worried about being caught lying about being a magician, but it's the other way, they won't believe he's anything but a magician if he shows off his fire spirit. So he can't say "it's not fire magic, it's a fire spirit." Bah humbug!