It's the holiday season once again, which means you'll see people celebrating Christmas, Hanukkah, the Solstice, and a few more. For some reason, over the last few years the misconception that people are no longer allowed to say "Merry Christmas" has become a hot topic. Well, folks, I have some news: You have literally always been able to say it.
If you've had an encounter with someone who may not celebrate Christmas, and they have yelled at or belittled you for saying "Merry Christmas", I'm sorry, that's really unfortunate and awkward. However, that does not give you the excuse to demand that Christmas be the only holiday recognized because then you're doing the exact same thing. I also see more people being criticized for not saying "Merry Christmas" than for saying it, so I'm guessing your experience is few and far between (though that doesn't make it any less valid).
I do celebrate Christmas, but for the most part, I say "Happy Holidays", and I'm glad to stick to that because a lot of times, I don't know what someone may or may not celebrate. It also incorporates New Years, and it's always nice to wish someone a happy new year. But if someone says "Merry Christmas" to me, I'll wish them one back. If someone says "Happy Hanukkah" to me, I'll wish them one back. And so it goes. There is no reason to be offended over whatever holiday someone is wishing you, it's nice enough that they were wishing you well at all, so let's stop belittling anyone for what they do and don't celebrate.
Furthermore, businesses and corporations are not obligated to cater to your holiday preferences. Starbucks has gotten hate in recent years for their holiday cup if it doesn't say "Merry Christmas" on it, or if it only says it once, or if it includes other holiday celebrations. Let's keep in mind that Starbucks has no reason to cater to anyone, nor does any business (perhaps, unless they market themselves to do so).
It's really not necessary to be rude to someone who was just wishing you well. After all, they didn't have to say anything in the first place. Please try to remember that there is no "War on Christmas". You were never told to stop saying it, you were just asked to remember that other holidays and traditions exist too.
With that said: Happy Holidays!