There is plenty of room for cultural diversity in the world, but not within a single country. A Tower of Babel has no culture. A person cannot be a Christian one day, a pagan the next and a Muslim the day after. All Americans have a huge stake in Christianity. Whether or not we are individually believers in Christ, we are beneficiaries of the moral doctrine that has curbed power and protected the weak.
Power is the horse ridden by evil.
In the 20th century, the horse was ridden hard. One hundred million people were exterminated by National Socialists in Germany, and by Soviet and Chinese communists simply because they were members of a race or class that had been demonized by political authority. Power that is secularized and cut free of civilizing traditions is not limited by moral and religious scruples. Christianity's emphasis on the worth of the individual makes such power as Lenin claimed unthinkable. Be we religious or be we not, our celebration of Christ's birthday celebrates a religion that made us masters of our souls and of our political life on Earth.