I'll just say this. As someone who is very secular and not at all religious (I was raised Catholic, married a Jewish man, raised our two sons within Judaism and neither one thinks of himself as Jewish) I do find "Merry Christmas" greetings a bit odd and off putting. But I recognize I'm the one here who is taking them as odd and off-putting. And that those saying "Merry Christmas" to me are intending to convey good tidings for the season. And so I say "Merry Christmas" back. The same with any other seasonal greetings. I respond in kind.
We don't need to tell everyone our particular take on these things, nor do we need to know anyone else's. We just respond appropriately.
Oh, and in my experience, it's not that hard to be nice to others. 
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