For those of us who do not celebrate Christmas, knowing how to respond to someone who might be sincerely wishing us well is difficult. Here are a few principles that are important to keep in mind:
1. Most people wishing us well about Christmas have no idea that Christmas is pagan in its origin, and are often unaware that we do not celebrate it.
2. There is usually not enough time to respond to that person by giving a short explanation of either the origin of Christmas or the fact that you don't celebrate it for reasons of conscience.
3. To most, saying "Merry Christmas" or "Have a happy Christmas" becomes so commonplace, it is very similar to saying "Have a good day." Most people say it out of obligation or common courtesy.
4. To simply say "Thank you" to someone who has wished you such would not be wrong—given the circumstances. It thanks them for the sentiment, but is not responding "in kind" by implying you observe Christmas.