I am atheist and I do find it a little offensive when people tell me Merry Christmas. I am not offended when people wish me Happy Holidays, because I consider New Year's part of the holidays. I am, however, highly offended by the Christmas display in front of my small town's city hall. I have called about the display to see if it was purchased by the city, and indeed it was. I have already made arrangements to contact the ACLU regarding this. How dare a local government that is using MY tax dollars purchase religious displays.

Because religious belief, or non-belief, is such an important part of every person's life, freedom of religion affects every individual. State churches that use government power to support themselves and force their views on persons of other faiths undermine all our civil rights. Moreover, state support of the church tends to make the clergy unresponsive to the people and leads to corruption within religion. Erecting the "wall of separation between church and state," therefore, is absolutely essential in a free society.
We have solved ... the great and interesting question whether freedom of religion is compatible with order in government and obedience to the laws. And we have experienced the quiet as well as the comfort which results from leaving every one to profess freely and openly those principles of religion which are the inductions of his own reason and the serious convictions of his own inquiries.
-- Thomas Jefferson, to the Virginia Baptists (1808). This is his second use of the term "wall of separation," here quoting his own use in the Danbury Baptist letter. This wording was several times upheld by the Supreme Court as an accurate description of the Establishment Clause: Reynolds (98 U.S. at 164, 1879); Everson (330 U.S. at 59, 1947); McCollum (333 U.S. at 232, 1948)