So...wait. Lemme see if I have this straight.
1) the current Saint makes a key only visible to "selected people or the saint candidates".
2) the Saint then doesn't "select" the priest carrying the thing, making it invisible to him, too.
3) the priest puts the key in his bag, where it could get lost, being invisible and all, and then dumps his bag out in his guestroom, predictably losing his invisible key.
4) rather than trying to find the key, he decides "screw it" and goes outside to ask an albino who doesn't even seem all that mystical at all whether she knows where the key she literally couldn't have known about is.
5) he brings the girl into the lord's manor, where the air itself probably has ears, to tell her "congratulations, you found the key. You're a much better candidate than this family's dumb brat". Also, he doesn't bother checking to see if she can actually see the key, which is the entire point of his journey, preferring to just wave it around and pretend like his choice wasn't already made.
6) said dumb brat overhears and frames Liz for stealing "the priest's treasury key", despite the fact that it's not actually the priest's, nor was it ever stated what the key was actually for. As a bonus, she brings the one person most likely and able to call her out to witness the spectacle.
7) instead of giving a Luke-style "every word you just said was wrong", he lets the situation play out practically until Liz's execution before stepping in.
8) going back to him not checking if Liz can see the key, what would have happened if she just got super lucky with her guess? Or if she had some other power? "Let's ask Liz if she can see anything." "Uh...no...?" "...eh?"
9) then he punishes and demotes the entire family for their daughter's crime when it hasn't been shown that they were in on it, potentially leaving the entire county in a power vacuum, which history has shown to be a wonderful state to be in.
10) then he declares Liz to be the next Saint, despite the whole point of the exercise being to just find candidates, leaving aside whether or not he has the authority to declare that in the first place.