There’s nuance to it, for sure. In the case of religion, I think it can be either or neither. If someone makes a bad faith-based choice because they’ve been raised in a cult, or they’re a kid who has been fed a lot of religious doctrine but doesn’t have any real world experience, or they live in a super religious community and only know scary lectures about atheists or people of other faiths, that’s ignorance. They don’t know any better and don’t have many avenues to fix that. If it’s something like Steve Jobs doing spiritual juice cleanses instead of getting cancer treatment despite having all the best medical resources and doctors in the world available, that’s stupidity. If it’s somebody who has a religious belief but doesn’t let that affect their decision-making detrimentally (they believe in miracles, but they’ll still go to the doctor for treatment when they’re sick) then they’re not stupid or ignorant. They may not be totally right but that’s some other fallacy - being too optimistic? Rose colored glasses? - that’s less harsh than stupidity or ignorance.

Your question about believing without understanding is a good one. There are a lot of things people believe without total understanding or proof. I believe Canada exists even though I’ve never been there. I suppose the difference there is the possibility of learning and proving it, as well as common consensus - I could go to Canada, I can talk to Canadians, I can see that Canada competes in the Olympics and is labeled on a globe. There are lots of different ways to verify it, and if all of the maps in the world vanished overnight people would find Canada again. But if I believed in a new undiscovered continent named Lemmy then I’d have to insist on believing it’s there despite there being no Lemmy citizens, no flag, no presence on a map, and so on, which makes it a much sketchier thing to ‘believe’ in than Canada.