Last month the Supreme Court issued a pair of decisions that dramatically illustrated our current uncertainty about the separation of church and state. In McCreary County v. ACLU of Kentucky, the court ruled, by a 5-to-4 margin, that a Kentucky county courthouse had violated the First Amendment by posting copies of the Ten Commandments. By seeming to offer government endorsement of a religious code, Justice Souter wrote in the majority opinion, the display of the commandments “violates the central Establishment clause value of official religious neutrality.” At the same time, the court ruled in Van Orden v. Perry that a different Ten Commandments display, this one a monument on the grounds of the Texas State Capitol, was permissible. A different 5-to-4 majority declared that the monument, unlike the Kentucky display, “conveys a predominantly secular message.”

The seemingly arbitrary and subjective nature of the decisions, and the narrowness and mutability of the court majority, do not inspire confidence. Clearly, the Supreme Court is just as divided about the proper place of religion in public life as the country itself. The important contribution of “Divided by God” (Farrar, Straus & Giroux, 306 pages, $25) by Noah Feldman – an NYU law professor and Middle East pundit – is to show that this ambivalence is anything but new. The debate over the proper relationship between church and state may be unusually heated today, but the basic questions at issue are as old as the republic. In this fast-paced, informative survey, Mr. Feldman shows how American law has evolved over 200 years of increasing religious diversity.

Colonial America was populated in large part by religious minorities fleeing persecution – Puritans in Massachusetts, Quakers in Pennsylvania. As a result, the Founders were strongly committed to securing liberty of conscience in the new nation. With the so-called Establishment Clause of the First Amendment – “Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof” – they believed they had cut the Gordian knot that entangled church and state in England. As Thomas Jefferson famously wrote, “Our civil rights have no dependence on our religious opinions, any more than our opinions in physics or geometry.”

The central lesson of “Divided by God,” however, is that each generation’s settlement of the boundaries between church and state is doomed to be upset, as the country changes in unforeseen ways. The Founders lived in an almost entirely Protestant country, where the education of children was private or informal. In the mid-19th century, however, the rise of public schools and Catholic immigration combined to make religion once again a politically explosive issue.

The earliest public schools, Mr. Feldman argues, were designed to instill mainstream moral values, in large part through reading of the King James Bible. This seemed uncontroversial to most Americans, but many Catholics found it subtly oppressive and sought government support for their own denominational schools. In the 1870s Republican politicians like James Blaine and Rutherford B. Hayes played to their Protestant base with a proposed constitutional amendment to ban the government from funding “sectarian” schools. The clear implication was that only Catholic schools were sectarian, while the Protestant-inflected public schools were simply normal. “Nonsectarianism,” as Mr. Feldman writes, “was an ideology of inclusiveness that was fully prepared to exclude.”

This was an early example of the political dynamic that drives church state issues to this day. The majority always sees existing religious arrangements as neutral and unbiased, until a minority arises to challenge them. In the late 19th century, it was radical skeptics, influenced by Darwinism, who took issue with America’s public religiosity. Robert Ingersoll, the crusading atheist, added a new wrinkle to the debate over public schools: “It is not fair to make the Catholic support a Protestant school,” he wrote, “nor is it just to collect taxes from infidels and atheists to support schools in which any system of religion is taught.” Ingersoll did not make much headway; then as now, atheism was unpopular in churchgoing America. But Mr. Feldman sees the late 19th and early 20th centuries as the heyday of “strong secularism,” when it became respectable to argue on philosophical grounds against any and all forms of religion in American life.

In a similar way, a new regime of church-state relations – what Mr. Feldman calls “legal secularism” – emerged after World War II in response to a new religious minority, Jews. In the most closely argued passage of “Divided by God,” Mr. Feldman suggests that it was sensitivity to the situation of American Jews – informed, inevitably, by the experience of the Holocaust – that led the courts in the 1940s and 1950s to mandate a stringent new degree of church-state separation.

But in another cyclical irony, this determination to avoid sectarian oppression – which dominated American law through the 1970s and led to the prohibition of school prayer – itself came to seem oppressive to a new religious minority: fundamentalist Christians, or what Mr. Feldman more broadly calls “values evangelicals.” Emboldened in the years of Reagan and the Moral Majority, evangelicals began to insist that the suppression of religion in public life was itself a form of discrimination against their own beliefs. Thanks to judges like Michael McConnell – appointed to the federal bench by President Bush in 2002 – this critique of secularism is now making headway in American law.

The legal debate that Mr. Feldman describes between secularists and values evangelicals is still unresolved, as we can see from those dueling Ten Commandments decisions. At the end of his book, Mr. Feldman proposes terms for a truce, involving greater public hospitality to religious speech, combined with less governmental support for religious institutions. But the real value of “Divided by God” does not lie in this hastily sketched compromise. Rather, Mr. Feldman has done an important service simply by showing how each American generation has tried to reconcile the competing demands of private faith and public reason – so far, with a heartening degree of success.