In 112 C.E., the governor of the remote Roman province of Bithynia wrote to the Emperor Trajan for advice. How should he deal with the “depraved … superstition” of Christianity? “I … do not know what offenses … to punish or investigate, and to what extent.” Trajan’s response included the famously tolerant directive, “The Christians are not to be hunted out.” By contrast, in 1999 C.E., as Peter Irons reports in his recent book “God on Trial” (Penguin, 384 pages, $26.95) “The national ACLU … launched a legal campaign against Ten Commandments displays in schools, parks, and public buildings.” After “monitoring … newspapers for reports of postings … [they] collected reports that more than a dozen counties had posted the Ten Commandments on courthouse walls, and warned them of possible lawsuits if they were not removed.”

Mr. Irons’s dispiriting book describes five recent court cases that hinge on First Amendment provisions having to do with religion. His professed aim is to put a human face on the country’s conflicts over the issue of the separation of church and state, and, in the process, to make complex legal arguments accessible. He is only partly successful. “God on Trial” is readable, if plodding, when describing the sequence of events in each court case. But Mr. Irons, a self-conscious “people’s” historian, is impaired by his own ideology from forming a coherent picture of the issues involved.

The Founding Fathers wrangled over the phrasing of the First Amendment’s religious provisions. They ended up with somewhat loose wording: “Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof.” The first half of this sentence, the so-called Establishment Clause, grew out of the framers’ dual concern that the federal government might someday attempt to establish a national religion and/or might meddle with the already established religions of individual states. But the second half of the provision is equally important. Mr. Irons falls prey to a common misreading of the first half and fatally ignores the second.

For Mr. Irons, the five cases in “God on Trial” demonstrate how the Establishment Clause protects individuals who are offended by government-sanctioned religious displays or activities — displays of the Ten Commandments, a cross honoring Korean War veterans that stands on public land, the use of the phrase “under God” in the Pledge of Allegiance, prayers offered before high school football games, and the attempt to include the teaching of “intelligent design” in biology classes. Religious partisans on the other side, however, fail to see how these things threaten to establish a state religion. They believe that those prosecuting these cases are, in effect, using the Establishment Clause to prevent Americans from enjoying their First Amendment right to the free exercise of religion.

But while he can tell two sides of a story, Mr. Irons’s certainty that religion has no place in the public sphere blocks his path toward cogent analysis. When the Constitution took shape — and for the next 150 years — religion was a natural part of American public life. Even today, most boards of alderman open their sessions with a prayer, as does the U.S. Congress. The tablets of Moses are displayed in the Supreme Court itself because Moses was an early lawgiver and because our system of justice derives in part from the Decalogue.

Contra Mr. Irons, the framers of the First Amendment had no desire to remove the pacifying, unifying, and edifying effects of religion from the public sphere. The right to publicly express religious belief is as indelibly written into the Constitution as the rights to free speech (with which it overlaps), free assembly, and a free press. Religious activism engendered the American Revolution and the abolitionist movement, and was the potent force behind the civil rights movement. At present, much religiously motivated political activity — anti-pornography, anti-abortion, and anti-gay marriage — no longer appeals to the liberal palate. Despite Mr. Irons’s distaste, however, religion’s place in American life continues to deserve protection.

Mr. Irons’s biases taint particularly the early chapters of “God on Trial,” which provide historical background. Genuinely shocking is Mr. Irons’s cavalier deployment of invidious comparisons: “We tend to forget that Islamic countries that have enacted the religious law of the Koran into their criminal codes are separated only by time and distance from the religious moralists of the New England colonies.” Well, yes, quite a bit of time and distance. And other things, too — like behavior. Mr. Irons shamelessly likens today’s Sharia to yesteryear’s colonial governance, as if the countless routine executions for apostasy, adultery, and sodomy of the former can compare to the terrible but exceptional witch trials and Quaker executions of the latter.

In terms of religious fervor, it’s those presently hunting out the pious that bring the early New England enthusiasts to mind. Debbie Mason of Santa Fe, Texas, who called the American Civil Liberties Union to block the offering of a prayer before high school football games, exults,

When the Supreme Court ruled against the school board, oh, they were mortified. I would have loved to been there and seen their faces. When I heard about it, I just started screaming and fell to my knees, and I was like, Thank God, thank God.

Oh, for the restraint of a Roman emperor!

Mr. Solomita’s writing has appeared in many publications, including the Mississippi Review, the Adirondack Review, and Eclectica.