Nobody ever got saved because someone said “Merry Christmas” instead of “Happy Holidays.” This time of year, if you insist on making a point of saying “Merry CHRISTmas” in reply to “Happy Holidays,” be sure you follow up with explaining your insistence.
Though some may dramatically exaggerate the attack on Christmas, saying “Merry Christmas” is not so rare that anyone would care when you do. So, without an explanation, most will not recognize that you are focusing on Christ. At best, your noble efforts will go unnoticed; at worst, you’re obnoxious.
So, yes, please say “Merry Christmas” instead of “Happy Holidays,” but then make every effort to share the gospel. Make your focus on “CHRISTmas” exactly that: a focus on Christ. Not just a personal satisfaction in doing your part in battling political correctness (hint: you will lose).
Make your greetings count this Christmas season and be sure to EXPLAIN the reason for the season.