Don't be misled by the seemingly bland title—itself a pun, referring both to religious groups' encounter with the law and the tension between law's letter and spirit. The Spirit of the Law is no ordinary history of the church and state battles of the past century. Lemon v. Kurtzman, the Supreme Court's much reviled and much amended Establishment Clause standard, is not even mentioned in the text. Even if this were the book's only virtue, it would be cause for great praise.

As many readers will know, the First Amendment of the Constitution lays down two seemingly irreconcilable dictates: Congress cannot establish a religion, but it also cannot abridge believers' free exercise of religion. Like other scholars employing the hot new perspective known as popular constitutionalism (prominently associated with Stanford Law School dean Larry Kramer), Sarah Barringer Gordon draws not just on technical court decisions but also on popular understandings of the Constitution in recounting the past century's Religion Clause battles. (In addition to being one of the nation's finest legal historians, Gordon is a colleague and friend, but we haven't discussed the book in any detail, so misinterpretations are mine).

Gordon charts these battles through finely detailed studies of six different religious groups whose encounters with the First Amendment helped to shape the evolving Religion Clause doctrine. The first chapter features the rowdy early years of the Salvation Army. Boisterous, noisy, and often booted out of the towns to which they brought their campaigns, the Salvationists marshaled the Free Exercise clause in their defense in an era when it was interpreted quite narrowly. Chapter 2 chronicles the Jehovah's Witnesses' challenges to mandatory recitation of the pledge of allegiance, which the Supreme Court initially rejected in 1940, reversing itself three years later. Chapter 3 is dedicated to the Supreme Court cases wrestling with state funding of religious schools starting in the 1940s. Protestants and Other Americans United for Separation of Church and State (POAU) spearheaded a campaign against funding for Catholic schools, drawing the active support of many conservative Protestants—until the Supreme Court banned prayer and Bible reading in the public schools, which persuaded many that the Court had taken separation too far. Chapter 4 stars the Nation of Islam, which was scattered and disorganized until Elijah Mohammed went to prison as a conscientious objector, invoking the Free Exercise clause to defend the Nation's right to meet and worship in prison. Chapter 5 is devoted to evangelicals' challenge to secular humanism in American schools in the 1970s and 1980s. Gordon's focus here is not Francis Schaeffer (though he is discussed) or familiar Christian legal defense funds like the Rutherford Institute, but Beverly LaHaye and Concerned Women for America, and its embrace of litigation as the tool of choice for social change.

The final chapter, on a small liberal organization called Religious Coalition for the Freedom to Marry that advocates for gay marriage, seems at first not to fit the pattern. Unlike the other religious groups, RCFM was not fighting for Free Exercise rights or embroiled in debates over the Establishment Clause. RCFM was, at first, simply a religious voice advocating a secular cause, and its initial argument—that civil marriage should be open to all—had a secular cast. But by 2004, RCFM had reframed its argument in more religious terms, stating in an ad in the Boston Globe that "For us any Constitutional amendment that denied same-sex couples civil marriage rights would be a violation of our religious freedom."

It is not always clear in The Spirit of the Law how the six groups have related to larger trends in American religious life—whether Beverly LaHaye's embrace of litigation sheds light on the trajectory of evangelicalism more broadly, for instance, or how significant RCFM is to the ongoing debates on gay marriage. But Gordon offers compelling evidence that "religious commitments and Constitutional law were mutually constituitive" for many of these groups. One of many gems is Gordon's discovery that the Nation of Islam, inspired by its new constitutionalism, adopted its own constitution.

Gordon's painstaking research yields a trove of other fascinating finds as well. Some will remember that the Supreme Court first invoked the "wall of separation" metaphor in its 1947 decision in Everson v. Board of Education, which inaugurated modern Establishment Clause doctrine. But who knew that the phrase—which comes from a letter written by Thomas Jefferson—was quoted in a Catholic organization's brief defending the use of public funds for busing to Catholic schools? And how many could have correctly answered a Trivial Pursuit question asking for the denomination in which President Eisenhower was raised? Answer: Jehovah's Witness.

The structure of The Spirit of the Law subtly suggests that the era of strong evangelical influence in America culture may be at least temporarily over. The last and most current chapter is devoted not to evangelicals but to more theologically liberal Christians and non-Christian groups. The Spirit of the Law—along with books such as The Religious Left and Church-State Relations, by Steven Shiffrin, another self-described mainline Protestant—may augur a revitalization of mainline Protestant legal scholarship. At this particular moment, their work is where the action is.

David Skeel, a law professor at the University of Pennsylvania, blogs about Christianity, law, literature, and other topics at lessleast.com.

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