An important fact that the modern-day religious right does not grasp is that the principle of church-state separation, which they revile so bitterly, was enacted for their benefit. It is probably not a coincidence that in Europe, where many nations still have established churches that are a legacy of earlier ages, church attendance is dropping precipitously, whereas in the United States, which has always had a vigorous tradition of separation, churches and sects of all kinds flourish and attendance and interest remain strong. Nevertheless, the partisans of the religious right have never been the kind to show gratitude, and usually harshly denounce victors in church-state lawsuits as atheists. However, this is not always the case. In fact, a significant number of famous First Amendment cases have been filed by religious people who objected to other religious people's religions being forced on them.
For example, take the 1890 case State ex rel. Weiss vs. District Board. In this decision, the Wisconsin Supreme Court ruled that teacher-led Bible readings in public schools, as had previously been the policy of the Edgerton school district, were unconstitutional. Were the plaintiffs atheists? No - they were Roman Catholics, upset that the Bible version being read to their children was the King James Bible, which they considered to be incorrect, incomplete and full of errors, as opposed to the authorized Catholic translation.
Similarly, in the 2000 case Santa Fe Independent School District v. Doe, the U.S. Supreme Court ruled unconstitutional a policy of student-selected, school-endorsed sectarian prayer at high school football games in the town of Santa Fe, Texas. Again, were the plaintiffs atheists? No: they were Mormon and Catholic families whose children were being harassed and bullied by the right-wing Baptist majority in the schools - not just by other students, but even by school administrators. (This harassment was so severe that the courts took the step of allowing the plaintiffs to file their lawsuit anonymously. As witness to the depths of hostility the religious minorities in Santa Fe endured, a federal district court was later forced to issue an order threatening school officials with criminal contempt if they did not cease their attempts to find out the identities of these students by means of "bogus petitions, questionnaires, individual interrogation, or downright 'snooping'".)
In another church-state case still making its way through the federal court system, Winkler v. Rumsfeld, two prominent Illinois religious leaders - a Methodist minister and a Jewish rabbi - are challenging a 1972 act of Congress that allows the Pentagon to spend government money to fund the annual Boy Scout Jamboree, since the Boy Scouts describe themselves as a religious group and openly discriminate on the basis of religion.
Many mainstream Christian groups have a long and proud tradition of defending the First Amendment. Americans United for Separation of Church and State, for example, was originally named "Protestants and Other Americans United for Separation of Church and State", and its founding members included a Methodist bishop and the president of the Southern Baptist Convention (source). Its current director, Barry Lynn, is also a minister in the United Church of Christ.
And then there are the many cases where the American Civil Liberties Union, the one group probably despised and slandered by religious conservatives more than any other, has fought and won to protect the First Amendment rights of Christians and other religious groups. For example, they sued in December 2005 on behalf of Joseph Hanas, a Catholic who was convicted of a minor drug offense and sentenced to a residential outreach program run by Pentecostals, who confiscated his Bible and told him that Catholicism was "witchcraft" and he would have to convert or else go to prison. They protected the right of a Christian church to conduct baptisms in a public park. They successfully argued that potential jurors could not be rejected for holding religious beliefs. They defended the right of a high school student to use a Bible passage as her yearbook quote. They defended students who were punished for distributing religious literature in schools. They have defended the rights of street preachers to use public sidewalks. In every single case, it was the First Amendment and its guarantee of religious liberty and free expression that protected the rights of these religious people from oppression by the state and by other religious groups. (Hat tip to Dispatches from the Culture Wars for many of these links.)
Finally, I offer for consideration an excellent case story for why we should value the First Amendment. Of all the unlikely places, this story comes from an evangelical Christian, Gary Christenot, writing for the extreme right-wing site WorldNetDaily (link). He relates the story of being assigned to an Air Force base on the Hawaiian island of Oahu, in a town heavily populated by Asian immigrants. Attending a high-school football game, he stood for the announcement of a pre-game invocation, and was shocked to realize the prayer was being given by a Buddhist priest. His account of the paralyzing confusion and shame he felt after experiencing pressure to conform to a system of beliefs different from his own is a letter-perfect account of the hostility experienced on a daily basis by religious minorities in a society where the government does not staunchly defend separation of church and state. I encourage reading the whole article, but will quote some excerpts:
We were frozen in shock and incredulity! What to do? To continue to stand and observe this prayer would represent a betrayal of our own faith and imply the honoring of a pagan deity that was anathema to our beliefs. To sit would be an act of extreme rudeness and disrespect in the eyes of our Japanese hosts and neighbors, who value above all other things deference and respect in their social interactions. I am sorry to say that in the confusion of the moment we chose the easier path and elected to continue to stand in silence so as not to create a scene or ill will among those who were seated nearby.
...The point is this. I am a professional, educated and responsible man who is strong in his faith and is quite comfortable debating the social and political issues of the day. Yet when placed in a setting where the majority culture proved hostile to my faith and beliefs, I became paralyzed with indecision and could not act decisively to defend and proclaim my own beliefs. I felt instantly ostracized and viewed myself as a foreigner in my own land.
There is a valuable lesson here for aspiring theocrats. The people who support eviscerating the First Amendment are invariably confident, with the blind confidence of the excessively religious, that their belief system would come out on top. But history teaches something very different: given time, religious minorities frequently become religious majorities, and when this happens, mechanisms put in place by the former majority to oppress other believers may be used to oppress them in turn. No two religious sects agree on everything, by definition, and any coercive intrusion of religion into government that some believers find congenial will have aspects that many other believers consider unacceptable, which is precisely why all such intrusions are constitutionally prohibited. The First Amendment was not put in place by America's founding fathers to oppress religion, but to protect the religious freedom of all sects alike.