I do the same. On the off chance that they are very obviously another religion I might say happy holidays. I was once stuck in Israel for a week while my C-130 got fixed. On Friday evening and Saturday, the Israeli's, from the guy that served you a Shawarma to the cab driver to the tour guide would say "Shabat Shalom" which means something to the effect of "peace be with you on this day of rest". I happen to be Christian, and Sunday is my day of rest. But these people were giving me well wishes in their custom. If the whole world would worry less about the details and realize the good intentions, we would all be a lot better off. Merry Christmas is wishing someone else a happy Christmas season, and happy holidays does the same thing.

So I just say Merry Christmas, as is my custom. If Delta tells me to say something else, I will do it, because this job is just too good.

Merry Christmas!