I repeat my Christmas message for Americans of all types.
In the U.S. we celebrate two unrelated feasts on the same day. One of them, called Christmas, is a religious holy day when Christians celebrate the Nativity - the assumption of human form by God, facilitating the forgiveness of sins and the redemption of the human race. We celebrate the holy day with congregational prayer, family prayer, images of an infant in a stable...you know the drill.
The second feast is called, "Christmas." It is a secular holiday celebrating - more or less - the Winter Solstice, when the days starting getting longer marking the coming of spring. It is celebrated by decorating a tree, exchanging gifts, and that whole drill.
We are all free to celebrate both, either, or neither of these feasts. When someone wishes you a "Merry Christmas," they are usually referring to the secular holiday, and there is no intention to proselytize any religion or insult those who don't celebrate the religious feast.
Deal with it.