Sure you can just say a Merry Christmas because it's what you believe, and so be it - there obviously is no malicious intent in your saying it and people who act like there is have a problem. However, I don't think saying a blanket Happy Holidays is a step in the wrong direction, either. I think wishing someone happiness on whichever holiday they celebrate, in whatever it is they choose to believe in, whether it differs from what you do or not, shows some respectfulness and consideration. And frankly, what I find most wonderful about the holidays are the different traditions and celebrations and the beauty in all of them. It's the kindness that matters, not the package it comes in.