I'm atheist and have many atheist friends. I don't know a single one of them that are remotely offended by the greeting "Merry Christmas." It helps to understand that many of us also celebrate Christmas, but as a secular holiday, not Christian one. Why would we be offended?

The real issue is a fake "war on Christmas" touted by certain media outlets intended to divide us and make liberals and atheists the enemy of Christians. That's the real war on Christmas - divisiveness where it doesn't exist at a time of year where we should be putting down our verbal weapons. 

Whether you greet me with "merry Christmas, happy solstice" or "happy holidays" - I'll just happily return the greeting.