A bit late to this thread. I'm an atheist because I reject the claims of theism as they have failed to meet their burden of proof. I do not "believe there is no god" for any value of god or gods.
Christmas is completely secular. Which parts of the celebration come from the Bible? Most believers don't know how many wise men there were - or whether they were kings. Most believers don't know that the gospels all describe different events and scholars believe that all the baby Jesus narrative was added to the anonymous story generations after it was written, in very high-form Greek, 60-150 years after the events portrayed in the story. Most believers believe the gospels were written by the names given to them.
The RCC fought for many years to block the translation of the bible into other languages so they could continue to dictate what it said rather than let people read it. Which was probably wise as the god of the Bible is clearly massively immoral.