Everytime someone in the shops wishes me happy holidays at the counters, I always respond with a blatant "Merry Christmas". I don't care who it offends, in fact, I think every Christian should be downright blatant with our faith. Because that is something that goes against us - we are not as obvious as alot of Jewish people or many other religious group, short of wearing the crucifix.
I hope you are sure to say it in a hostile and angry manner as your post seems to convey. What a perfect way to display your faith!