Big topic in the office - and elsewhere - is it appropriae in a culturally diverse Australia to wish colleagues a Merry Christmas or should we wish them a sanitised "fesitive season" greeting?
I've come to a pretty simple conclusion that we should stick to Merry Christmas especially if connected to wishes of peace and joy. In reality we only have a "festive season" because of Christmas - so wishing people a happy "festive season" is referentially the same. Secondly wishing people a Merry Christmas doesn't mean that one is necessarily tieing anything to the date - after all we know the date of Christmas was chosen to coincide with pagan mid-winter festivals, not because it was the date of the birth of Jesus of Nazareth.
Finally, even when we practice religious tolerance we don't, and should not, outlaw prosletysing. If someone wants to say Merry Christmas because they are a believer, the fact the other person isn't a believer shouldn't be a reason to stop them. And the person receiving the greeting, even if of a different faith, should not take offence - the wishes are well intended and are offered by someone who cares about you. Only a particularly insular christian would not wish a Merry Christmas to non-believers.