By chance, I was asked about my religious background.
Like most Japanese, I tell I don't believe in any particular religion when I have a yes-or-no question.
But it doesn't necessarily mean they are atheist.
(Of course, the situation in U.S. is also complex. For example, this article looks interesting.)
I tried to explain nature worship in Japan and then the distinction between guilt cultures and shame cultures proposed by Ruth Benedict, but my explanation was not so good.