Who believes the United States of America to be a Christian nation? Serious question - I'm curious and also as to why that's even of issue?

Depends on your definition of Christian nation. If you mean a theocracy, no. If you mean a nation founded on Christian principles, yes.

So if I'm not a Christian do those principles apply to me? They sure didn't for my gender or racial mix until last century.

Trial by jury, female judges, 10 Commandments stuff like do not steal, do not murder,… foundations of law apply to everybody.