Most Atheists define Atheism as “lack of belief in a God”, which seems like it applies here. Agnostic is usually defined as “lack of knowledge of a God”, which also works here, so both Agnostic and Atheist.
It makes sense when you look at Theist (with belief in a God) and Gnostic (with knowledge of a God) that adding an A before it just means “without”.
I can easily see this being a safety issue. You don’t usually want employees wearing stuff that could anger other employees or customers, no matter the reason.