I find political correctness revolting, and especially so this time of year. My personal beliefs should not matter to you, as yours do not matter to me.
It IS Christmas. That is what the holiday is called. It doesn’t matter if you are a believer or not – it is still called Christmas. Need another example? July 4th is NOT called “Firecracker Day”, or “Beer and Wieners Day”. It is called “Independence Day”. Stop calling them “Holiday Parties”, and the “Holiday Tree”. They never WERE those, and they never will be.