that's part of the issue with dealing with OP protagonists in these kinds of settings, inevitably they run into the edge cases of their world's etiquette. Myne's skillset goes against the religious doctrine of the nation, but also he isnt really doing anything "wrong" per se, and also it is a bit ridiculous if anyone did find out and try to argue against him using his skill considering he got it from the same god they all worship. The guild master knows Myne is suspicious and wants the truth so he can trust him, but Myne is also aware he would be a pariah if anyone else knew the truth of his situation.