Look, if you believe in the Christian God, you believe in an all-powerful being. So do I really think that God is going to be somehow weakened if the person helping me find the right size sweater said "Happy Holidays" rather than "Merry Christmas" when I walked in the store? No, no more than I think that the 1954 addition of "under God" to the Pledge of Allegiance caused God to care any more or less about America. Nor do I care if children in a public school are asked to sing the Dreidel Song to go along with Silent Night at a Solstice concert, or if the school has decided to not mark any of the holidays that occur in December. There are options other than public school, if it is that important to you that your child be exposed to no other viewpoints than that of your own tradition, or if you feel that religion is a necessary component of your child's education.
What is important to me is the sentiment. Having some bored silly store greeter flatly wish me "Merry Christmas" does nothing for me. But the little boy down the street who gave me a big hug and wished me Happy Channukah filled me with delight, and made me feel loved. The genuine wish of joy from another person strengthens my faith. It does not diminish it.
And if people are concerned that the Christmas Spirit is being diminished, maybe, rather than insisting that their City Hall put up a Nativity in December, they might insist that their government feeds the hungry, shelters the homeless, and clothes the poor year round. They might educate their children in the values of compassion and understanding, of striving for social justice. They might live as Christians in actuality, rather then relying on symbols and empty phrases to impress other people with their righteousness.
The title of this post, incidentally, comes from an event that reminds me of true Christmas spirit, the Christmas Truce of 1914. Here is an account, and here is a song about it.