I was going to stay out of this, but I have to chime in. Nancy324 did a great job of being respectful yet showing her point of view, so I'm gonna try to be nice here and not offend anyone.
I'm an atheist, as is my husband, and our son is being raised that way. However, if he wants to find religion when he grows older, that is fine by me. Both DH and I were raised in Catholic families who celebrate Christmas. We celebrate Christmas in our house, but not as the birth of Jesus, but more as a family togetherness holiday. So, even though I am not religious by any stretch of the imagination, I tell people to have a Merry Christmas, and I don't mind that people tell me that, either. I'm not big on being told Happy Holidays, but I can see where others don't mind it, as it is more inclusive. To me it is Christmas season, but that's how I was raised. I could see to many others how it could be Hanukkah, Kwannza, whatever season. To me, Happy Holidays covers from Thanksgiving to New Years, where Merry Christmas covers a few days or a week. I think people just say whichever they were brought up, whatever is natural to them, and truly don't mean to offend anyone, at least that is the case to me.
Honestly, most of the time, I don't say it strangers b/c who knows what my tick someone off these days, and they decide to go crazy. Unless I know someone, I don't say anything until they say it first. I someone tells me Merry Christmas, I'll tell them the same back. If someone tells me Happy Holidays, I'll normally tell them Merry Christmas back, if its close to Christmas, or if its not, I'll tell them Happy Holidays. However, if someone tells me "Merry Christmas, god bless you" I'll tell them "Merry Christmas and no, thank you."
That being said, and although I don't religiously agree with what Tami posted, it is a very nice way of putting things, and the spirit/sentiment is right.
See, I said that nicely. Go me
Nan