Actually, Xmas is short for Christmas. The letter X comes from the greek letter chi, which is the first Greek letter in Kristos. Χριστος (Greek for "Christ")

http://en.wikipedia.org/wiki/Xmas#History

http://en.wikipedia.org/wiki/Labarum


Christ has been abbreviated thusly for millennia. It is not part of ANY deliberate attempt to take the Christ out of Christmas, which I'm sorry to say, is an imagined plot. Any concerted effort to take Christ out of Christmas would fail, as there are a billion Christians on the planet who might care about it, and far fewer atheists and non-theists who find Christ offensive (the insignificant minority of them).

Most of us non-theists actually appreciate and admire Christ, as a leader, philosopher, and spiritual man. The bottom line is that we just don't think he's God incarnate, or that there is one. That doesn't mean we hate him. In fact, I've made it my personal duty to be more informed about Christ than the average Christian. I have several Bibles and am aware of cross-cultural and non-Christian perspectives on Christ. You'd be surprised how many non-Christian groups think Christ was a damn swell guy.