The state of Kentucky is providing major tax incentives for the development of a Bible-based theme park called Ark Encounter. The featured attraction at the park will be a rendition of Noah's ark, complete with an on-board menagerie. The park also will include a Tower of Babel and special effects depicting the ten plagues and the parting of the Red Sea. The park was conceived by, and has an ownership relationship with, the same Christian ministry that built the Creation Museum, also in Kentucky. That museum has evolution-denying exhibits that depict humans and dinosaurs living together on an earth that God created in six days. The tax incentives, which will enable the developers of Ark Encounter to recoup as much as a quarter of their investment, have raised the issue of separation of church and state, given that it constitutes major public support for a facility that promotes a specific religious message. The governor of Kentucky downplays the issue and says the public support is all about creating jobs.

The theme park in Kentucky represents one of the latest instances of erosion of the constitutional prohibition on state support of specific religions or religious dogma. Other examples in recent years include the attempts by some state and local boards of education to shape curricula to make them conform to specific religious beliefs such as creationism. Another example has been the influence of evangelical Christian doctrine at the U.S. Air Force Academy, as exemplified by a religious banner that the football coach displayed in the locker room.

Part of the American political right is promoting the breakdown of this constitutional principle. Kathleen Kennedy Townsend writes of one manifestation of this: criticism by Sarah Palin of the late President John F. Kennedy for downplaying rather than flaunting his religious faith. Townsend is right that Palin's version of the promotion of faith in public life lays out a dangerous path in which political leaders come to be judged by their religiosity, and religiosity in the sense not just of some general spiritual wholesomeness but of specific beliefs and of specific deeds and policies in accordance with those beliefs. The judging will be done not by a broad public consensus but instead by self-appointed public apostles like Palin, as suggested by Ross Douthat's observation that religiosity in America has become increasingly concentrated in a more educated (especially evangelical Christian) church-going elite.

I have long studied international conflict and terrorism, especially in the Middle East. The subjects of my studies are filled with reminders of how the more that religion is infused into public life, the more that mankind suffers. True, there are many conflicts in which religious identity serves more as an organizing principle than a prime mover, and in which some amount of conflict would be present in any case. But any reflection even just on the conflicts and strife in this one region leads to the conclusion that if religious belief could somehow be erased from minds and motivations, a huge amount of bloodshed would be erased with it. This may be most apparent to Americans in the religiously rationalized terrorism that has become a major concern. But we see it as well in, for example, the competing beliefs of Muslims and Jews (not to mention Christians) regarding how their respective deities (or respective versions of the same God) allotted the same piece of real estate to each community. Then in Iraq there are Shia and Sunni—Muslim and Iraqi on both sides--killing each other. The list could go on and on.

Religious beliefs inherently contain the seeds of intolerance, and thus of conflict and extremism, in ways that most secular belief systems do not. If one believes one's dogma comes from divine will and providence, it can less readily be compromised in good conscience than beliefs of more mundane origin. And those on the other side of a conflict can be seen not just as in opposition but as evil.

Religious belief, because it deals with the unknown and unknowable, must quite literally be a matter of faith. And questions of faith, because they cannot be resolved through public debate, appropriately dwell in the realm of the personal and the private. Once injected into the public realm and more specifically into matters of state, then they become one more form of the tyranny over the mind of man against which the deist Thomas Jefferson swore eternal hostility upon the altar of God.

This is why the principle of keeping religious doctrine out of the affairs of state—the concept commonly called separation of church and state—that Jefferson and the other founding fathers bequeathed to us is so important and so precious. It is fundamental to the free thought and rationality that in turn underlie so much of what makes the nation strong and great.

Reassuring indications of the strength of this principle are still to be found. It was too bad that Kennedy’s Catholicism was an issue at all in 1960, but it did not prevent him from being elected. Today, most memories and assessments of Kennedy and his presidency, positive and negative, have little or nothing to do with his religious identity. Another reassuring instance was the reaction to Elena Kagan’s replacement of John Paul Stevens on the Supreme Court, resulting in a court consisting entirely of Catholics and Jews, and for the first time not a single Protestant. This religious side of Kagan’s ascension to the bench was noted, but one cannot really say it became an issue, or at least not enough of a one to become a subject of mainstream discussion.

Despite these reassuring episodes, the principle is not just precious but also fragile. The threats to it are alive, as suggested by Palin’s comments about Kennedy. The seeds of intolerance and extremism can exist in religious dogma within the boundaries of the United States, just as they can elsewhere. We perhaps have gotten a glimpse of the extremist possibilities in recent times with violence at abortion clinics, which probably is stimulated by religious dogma (as compared with other stimulants, such as resentment over U.S. policies) at least as much as is terrorist violence with an Islamist label, such as Nidal Hasan’s shooting spree. And even without violence, any tyranny over the mind of man, made possible when the doctrines of revealed religion are mixed with the powers of the state, can be debilitating to the United States in the same way, even if to a lesser and less visible extent, that it has debilitated other countries.