How is it possible that anyone can be offended by the words "Merry Christmas"? Having been the only Christian working for a company of Jews, there were numerous times people wished me well during Jewish holidays and I didn't get all upset. I was amused if anything. I don't celebrate those holidays, but I certainly didn't take offense to someone suggesting that I enjoy them anyway!
If you are an atheist or an adherent to a non-Christian religion, you already don't believe that Jesus was the son of God. So, the fact that someone wishes you well on his birthday should be the same as someone saying "Happy Santa Day".
For Christians, the birth of Jesus was the ultimate act of love by God. He sent his own son to live as a man and pay the ultimate penalty for ALL mankind. How can such love and sacrifice be offensive? If you find anything in the life, death and resurrection of Jesus offensive, perhaps you are the one with the problem.