An atheist majority locality (think of such a community as a possible future development) would violate the non-establishment clause if its public schools posted "religions are false primitive superstitions and gods are fictions" posters in classrooms and its local government directed teachers to lead students in a government specified "nation with no gods" patriotic ritual at the start of each school day. That is the way law must be to be just. Just = even handed = no double standard = no discrimination = equal protection for all before the law. Free exercise and non-establishment thus requires government neutrality between religion and non-religion, between theistic (mono and poly) theologies, atheistic theologies, and deistic and atheistic atheology. Accordingly, the Supreme Court recognizes that when a person sincerely holds beliefs dealing with issues of "ultimate concern" that for her occupy a "place parallel to that filled by . . . God in traditionally religious persons," those beliefs represent her religion. The various theisms, deism, atheism etc. are equivalent to "religion" for purposes of First Amendment jurisprudance (see McCreary County, Ky. v. American Civil Liberties Union of Ky., 125 S.Ct. 2722 (2005)).
People who argue against this equitable 1st amendment interpretation are usually advocates of government discrimination in favor of citizens who adopt the majority religious beliefs and against everyone else. Not coincidentally, most such people share the majority beliefs that they want the government to favor and oppose government favoring other contrary beliefs. They tend to falsely label neutral (silent) government as being antagonistic to (discriminating against) their beliefs. On the basis of that skewed perspective some people incorrectly deny that neutrality is possible. Virtually everyone on the religious right adopts some form of this unbalanced perspective. It is true that government cannot be literally 100% neutral all the time in all contexts because some religious practices are harmful to others. Religious beliefs do not decriminilize forced "marriage" of children to an already married older person or refusal to obtain life saving medical treatment for ill family or friends. Similiarly, people are imprisoned for speech that directs someone else to commit a crime as part of a criminal conspiracy. Exceptions to first amendment protections are sometimes required to protect people from harm. Such exceptions do not demonstrate that 1st amendment rights, including non-establishment neutrality, are impractical concepts that are impossible to realize and thus should be abandoned.