I'm an atheist, but until the past few months I never had the slightest problem saying or hearing "Merry Christmas." For one thing, it's a national holiday as well as (for some) a religious one; for another, why should anyone take offense that someone is wishing them something good? And so I thought that those atheists (and others) who took offense were just being silly.
However, Trump has changed all that. He has made "Merry Christmas" into exactly what those silly atheists always thought it was: a deliberate affront to people who don't agree with Christian beliefs. When I hear a stranger say "Merry Christmas" now, I can't help but think there's a possibility they might be Trump supporters who are really saying "**** you, atheist." Of course, there's a much greater chance they're wishing me a nice holiday, but the phrase is now forever tainted. For that reason, I don't say it and I don't respond to it, and I'd encourage thoughtful people to do the same, as a rebuke to Trump and his garbage supporters.