????? even irl back in the medieval ages not "everyone" believed in god, for most it was tradition + would literally be killed as a heretic if they didnt claim they were. you also have to consider it depends what god your talking about, the Christian god for example even today is only worshipped by about 1/3 of the world, not even half. and a millenia ago there were many pagan religions as well, so no, its not unrealistic for "not everyone believes in god"
besides, its a fictional world, why are you basing their religiosity on the real world? do you think religion is a given? that humans by nature must believe in god and therefore its weird when a fictional world doesnt?
also we don't know the religious makeup of this world, maybe the other half believe in a seperate polytheistic religion that doesnt care if people dont believe in the divine hence making it safe to say you dont believe, or maybe they believe in divine fairies like nymphs instead etc... we literally just dont know, also this is one duchy, we dont even know if the one god church has more influence elsewhere, again, we dont know
anyway, apologies for the rant, its just weird to me how people can complain about a fictional world not being as religious as possible.
it's not so much that it's not as religious as possible, so much as that its take so far is a little inconsistent. If the church has enough power to hold witch trials, that only works if they hold enough of the people's faith to get away with it. If what another person who replied to what I said is true, and that the story somewhere mentioned that only about half the population believed in the church's God, then there's no way, no matter how powerful the organization, that people could be killed for what half the population believe to be lies and fairy tales, without massive violent riots. Sure, the country/church could put them down with force, but it'd be a big deal, and should be told about in the story. Thus, since witch trials are held, those opposed to the trials should be a weak minority, heathens who would defy or doubt the word of what is clearly the nation's diety, a crime clearly punishable by burning at the stake in this setting. Thus, one doesn't simply proclaim to not believe in the nation's God when the population is clearly, at least fanatical enough to tolerate one being burned alive for other forms of heathenism (like being a witch). Even if the person she said it to was saved from that very fate.
Also, this is a European setting with a church very clearly modelled after Christianity. Sure, you could write a story with such a premise and then make the country's religious details deviate significantly from what any similar real country was like in similar time periods, but it should be done in a believable and thoughtful way or something. Idk, like, I get not every world building detail can be super thoughtful, but it's kind of a shame (in my opinion) that it's so common in these kinds of stories for depictions of religions (particularly Christianity based ones), to be so half assed and particularly, negative.
To be honest, if the other half of the population did believe in some other interesting religion like you said, that would be pretty cool I think, and more thoughtful, but the author hasn't said anything about that.