yellowtamarin wrote:
The difference between saying "God bless you" and "Merry Christmas" is that the first is habit, as puddingmouse said, i.e. it often just slips out of your mouth when someone sneezes and you don't think about it, while the second is a deliberately spoken sentiment that has thought and purpose behind it. You don't just accidentally say "Merry Christmas".
While the Christmas holiday still has a Christian element in its name, many of its traditions have very little to do with Christianity and are inherited from the Germanic midwinter-feast or the Nordic jul-feast. It's perfectly possible to celebrate Christmas without sticking to the Christian elements of the holiday. Atheists celebrating Christmas will still have a nice dinner with family, put up a tree, give presents, etc... So if we're going to be celebrating the holiday, why bother with avoiding its name. It's not practical at all.