I used to work for some company located in a pretty conservative part of the US. People were fairly open about believing in the Christian god, going to church, and sending their kids to Sunday school and that kind of thing, but not really a culture of getting together as coworkers to worship or something like that. People lived in different parts of towns and went to their own denominations/churches. I do think if you were an atheist employee, you would feel pretty uncomfortable being open about it, and if you were a Christian you would feel super comfortable saying it.
I mean this was a part of town where not showing up to church itself was a sin so even amongst Christians saying that you didn't go or only went on Christmas/Easter would be a taboo thing to say. And during Chick-fil-a's gay marriage debacle there was a line of employees standing outside to order food during "support Chick-fil-a day."
Now I work in a super liberal city and nobody talks about their own religion, if they have one, or maybe at most would make vague references about it. People who are atheist or nonreligious would be pretty comfortable saying it.