Just tell them that even though you don't believe in God, that you still enjoy the spirit of the holidays, and that you still want to celebrate. Remind them that Christmas is more about family, and generosity, etc. more than believing in God. Maybe if there are no young children in the room you could even say "well you don't believe in Santa, you still celebrate Christmas" in addition to that to emphasize the fact that Christmas is more about kindness than religion and scripture. Remind them that 95% of US citizens celebrate Christmas despite the fact that only about 70% are actually Christian. After they calm down, just reason with them in this way. If they continue with their current attitude, then you can start tearing their hearts out, because the least they can do is meet you half-way.
"Sisters, you know only the north; I have traveled in the south lands. There are churches there, believe me, that cut their children too, as the people of Bolvangar did--not in the same way, but just as horribly. They cut their sexual organs, yes, both boys and girls; they cut them with knives so that they shan't feel. That is what the Church does, and every church is the same: control, destroy, obliterate every good feeling. So if a war comes, and the Church is on one side of it, we must be on the other, no matter what strange allies we find ourselves bound to."
-Ruta Skadi, The Subtle Knife
-Ruta Skadi, The Subtle Knife
