That may be so, Nigel, but it's a good bet that most people who use "Xmas" use it as a convenient shorthand, or just a lazy one, and are unaware of that Greek origin. As an atheist I have no dog in the fight as to whether it's offensive or not to believers, but a quick google revealed that there are plenty of Christians who find it offensive (try Quora for example). As it takes about 0.568 seconds longer to write the word in full, and as I have many Christian friends, I'll carry on choosing to avoid the short form.