> You don't have to lie to people. You can go to church and not believe. I know several people that do that.

Interesting. In my (predominantly Catholic) country, people do that for weddings and funerals, it's widely accepted and in fact it would look radical to stay out of a wedding for not believing. But I don't think non-believers going to "regular" mass is a common thing, I think it would be seen as odd and probably not very welcome by the believers.

Also, one of the main prayers said in Catholic mass starts with "I believe in God, the Father Almighty". So consciously being in a group that says that looks literally like lying to me, even if it's a white lie.

It's not really a question of the church's "business". It's that I do not believe in God, and lying to people about something that central to their identity seems like a bad thing to do. I'm skeptical that I'd be accepted in the vast majority of churches as an open nonbeliever without continual attempts to "convert" me [1], particularly in the area where I live. I've heard of churchgoers and even priests/pastors who are atheists, but very few ones who are actually open about it to their congregation.

[1] Incidentally, it's not that I'm not open to changing my beliefs, it's that (in my experience) believers are aggressively awful at converting nonbelievers. In many cases they don't even accept that it's possible to be a nonbeliever, immediately falling into "angry at God" tropes.

I know of some people who go to church even though they don't believe, and it seems to work for them. For a lot of people, though, that becomes extremely uncomfortable. You say you don't have to lie to people, and maybe that's the case, but I suppose it depends on one's definition of a lie.

Given a prayer to a god one doesn't believe in feels dishonest for some, as does nodding along and letting people assume you share their beliefs when you don't. The foundation for church is supposed to be a shared theological belief, so it's not too difficult to understand why some might be uncomfortable joining when they know that they don't share the belief and have to go through certain motions to conceal this fact from their brethren.