I'm seeing you fighting the war against "culturally Christian" accusations again (good for you), & I think it's so wild how these other non-christians think they can decide that we're Christian without considering whether CHRISTIANS would claim us. I grew up a 100% secular atheist in a heavily evangelical area of the US. The Christians there hated me! I experienced ostracization, and discrimination from children and adults alike for not being Christian. They would certainly never have agreed that I'm in any way Christian, culturally or otherwise! So why do I have to submit to this idea of being a privileged "culturally Christian" atheist, when I grew up as an actively persecuted religious minority? It just makes no sense to me.
Not considering whether Christians would claim us is bad enough, but to not consider whether we consider ourselves as connected to Christianity in any way is even worse. Especially when we explicitly say we have no connection.
Because if even people who actually know us might get it wrong (many atheists are in the closet, after all) some random stranger on the internet isn't going to know us better than we know ourselves.
Someone described it once as this forced labelling as being the religious equivalent of forcing a gender onto a trans person. In both cases you're saying you know better than the person who they are and what they feel and believe. And in both cases it can be traumatic to put it mildly. I've been misgendered and called culturally Christian before, and I gotta say, it doesn't feel very different from one to the other.
And, unlike in the case of gender, when they force the label on you there's the added trauma that they're equating you to the people who have oppressed you and your fellow atheists for centuries. In my case there's the added disgust layer of being told that I, an Indigenous woman (from a tribe that prides itself in resisting white Christian missionaries no less) have to be Christian. Who tf knowingly says that to an Indigenous person???
I think that the insistent labelling of atheists raised secular as Christian comes down to three things:
Theist people feel threatened by atheists merely existing because it reminds them that not believing in a deity (even if not their own) is an option; that it's possible to be good (both in the sense of morality as well as well-being) without any god belief.
Theist people can't comprehend that atheists have their own culture; they think that atheists must have a culture void in their lives that obviously needs to be filled, and whichever is the majority religion where they live will fill that void.
They're either ignorant of the fact or ignoring the fact that different countries have very different ways of relating to religion in their country culture.
The first thing is very much a case of them making their insecurities our problem.
The second is why atheist parents often have a hard time getting custody of their children in court; judges often side with the religious parent because it's believed that atheists are lacking something necessary to raise children (which, obviously, we're not). A famous historical example of this was Percy Shelley, husband of Frankenstein author Mary Shelley. He lost custody of his kids and was expelled from university for being an atheist.
The third, in my experience, tends to be an issue most commonly seen with American theists. They assume that the way things work in their country is how things work everywhere, which is demonstrably not true.
In Brazil, for example, we use waaay more religious language in everyday colloquialisms but people don't really talk about their religion as much as they do in the US. The only times I've talked with my friends of 33 years about religion was when they were inviting me to big religious events in their lives. It's more of a private thing.
And if politicians in Canada were to go around mentioning God the way American politicians do? It'd at best be a scandal and at worst they'd have to step down.