The phrase "separation of church and state" (sometimes "wall of separation between church and state"), attributed to Thomas Jefferson and others, and since quoted by the Supreme Court of the United States, expresses an understanding of the intent and function of the Establishment Clause of the First Amendment to the Constitution of the United States. The First Amendment reads "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof.
The ACLU, in a letter to city officials, warned that displaying a Nativity scene and menorah violates the separation of church and state.
The article goes on to talk about how not everyone is Christian therefore displays should not be allowed in public. But not everyone is atheist, or believes in what the ACLU preaches, yet they feel their views and beliefs should take priority over anyone else's. This group threatens others if they do not do as they want, but I have yet to hear of a nativity scene threatening anyone.