I was christened but I would never refer to my first name as my christian name. I'm not a christian. We do not live in a christian country, simply because the vast majority of people here are not practising christians. Just because people choose to lie their socks off in a church when they get married, or when they name a baby, or when they die, does not make them christian. Sorry to burst your bubble.
I really do hope Julia has stopped using this term. At best it's inaccurate and at worst it's devisive.
I agree with you, quelle surprise, but I don't take the argument as far as you have. To me good manners is consideration for others and to me (yes just me and my take on things) that means not making assumptions about things such as marital status, food preferences, orientation to name a very small few. I try sincerely not to be dismissive of people, it's important to me and what passes for a conscience.
Having been told by someone desperate to flog me some jewellery that they don't ever think of a cross as a symbol of Christianity I should have given up then