By disclosing my personal preferences, I didn't mean to imply that everyone needs to do everything exactly as I do. If you have found that this approach works best for you, then full steam ahead and best wishes to you. If your friends have indicated to you that they would be offended by learning of your appreciation of Christmas in the form of a card and of your extension of a sentiment of goodwill to them based upon your religion, and you wish to interact with them in this non-religious way then that's great if that's what brings you happiness.
I, however, have never told anyone that I would be 'offended' by anything at all and so I am baffled when I am sent cards that make no mention of Christmas or the purpose of the Holy Day. In my case, it seems to me that there are some who wish to remove the entire meaning of Christmas, and turn it into just another day off. Actually, I'm not one to parade my faith publicly and so most of these sorts of cards I get are from people who have no idea where I stand on Christmas one way or another. They are assuming that I am, like them, a PC liberal, and this I find unfortunate, that there is this arrogant assumption at play as well.
To people who would question this- would any of you like to receive a "Happy Ramadan! Praise be to Allah!" card this year?
I don't have any Muslim friends (at least to my knowledge) and so this hasn't come up, but I would welcome any card expressing friendship and goodwill. If it's expressed in the context of their religion, then so much the better.