By NAOMI SCHAEFER RILEY

Toward the beginning of "Down in the Chapel," Joshua Dubler describes a conversation he had with an inmate at Graterford Prison, a maximum-security facility about 30 miles northwest of Philadelphia. "I don't want to disillusion you," the inmate tells Mr. Dubler, "but a lot of these dudes just come to the chapel for something to do."

Mr. Dubler, at the time a graduate student at Princeton University, was far from disillusioned. He had come to Graterford to study the practice of religion in prison and quickly discovered that men do indeed head to chapel or make other gestures toward religious observance in part because there's nothing else to do. Or because they hope to impress the parole board or because they want to use the chaplain's phone to call home. But Mr. Dubler chronicles something remarkable at Graterford as well: committed worshipers who, at times, debate what religious belief is or should be.

As an Episcopal minister who comes to teach at the chapel tells Mr. Dubler (in the author's paraphrase): "People on the outside refuse to believe the sort of spiritual maturity guys in here have." One inmate, a murderer serving a life sentence, began studying Greek and Hebrew in prison and has by now, Mr. Dubler says, carved "a sizable path through the canon of late biblical and early rabbinic literature." Born a Protestant, the man has converted to Catholicism in prison because, he tells the author, it allows him to access "both the numinous and the mystical facets of religious experience."

Most of the winding conversations that Mr. Dubler records are more down-to-earth about religious matters. In one, an inmate says: "I see a lot of brothers who profess Jesus Christ as their personal Lord and Savior, and I can't look into their heart but they don't live it out in their actions, even when they're down here in the chapel. They don't live with the gifts." To which another inmate responds: "How can you say they're not using it if you don't know what God gave them? . . . You can't tell them what the proper expression of their gifts is! That's like Protestants who say you've got to speak in tongues. But I gave my life to God on December 11, 1991, and I ain't never spoke in tongues." Another inmate, listening in, adds: "You don't have to set yourself apart or anything like that. Because when it comes down to it, it's not what we do, it'swhhhhhh, whhhhhh"—at which point, Mr. Dubler tell us, he blows through his fist. "It's what we are." As the author summarizes the point: "Spirit is what we are."

Mr. Dubler's ability to capture these conversations—the differences among inmates, their changing moods and shifting positions—is nothing short of amazing. He spends most of his time in the chapel and its surrounding offices, with Protestants, Catholics, Jews, Muslims and Native Americans. (The chaplains don't like to go see the men in their cells because it seems too "embarrassing" for the inmates.) During the course of a single week—the book's chapters chronicle one day at a time—he attends various services and Bible studies and acts as a stenographer for impromptu conversations. His note-taking is dutiful and sprinkled with observations of his own. Of the Jehovah's Witnesses, he notices "the bureaucratic flavor of their biblical urgency." Of one of the chaplains, he notes: "On his more pessimistic days, [he] suspects the administration sees his staff as little more than affable opiate peddlers—feeding inmates false hopes that make them, if nothing else, better prisoners."

Along the way, Mr. Dubler looks at the history of Graterford, particularly at a period in the 1970s when "humanitarian innovations" were implemented and, coincidentally, the Nation of Islam expanded within the prison, and at a later period, when a raid found that the Nation was running, for all intents and purposes, a crime syndicate on Graterford's grounds. The number of Muslim attendees at services has been low ever since. Salafism, a strict version of Sunni Islam, is the dominant form of Islamic affiliation now. The influence of the Nation of Islam—the black-centered, American-born offshoot of Islam—has evidently faded. While Mr. Dubler, a secular Jew, hears plenty of talk about the vast Jewish conspiracy that has left all of the inner-city blacks in poverty, he doesn't here any talk about radical jihad.

At one point, Mr. Dubler describes himself accompanying the Catholic chaplain into the Restricted Housing Units, the home of lifers and, temporarily, of those confined to solitary confinement. He writes that one of the inmates there, a man who is in "the hole" for yelling at a guard, "teases me about my dissertation's lack of a thesis." Presumably, the author is often asked about his formal graduate-school writing project and has given a diffuse account of it. To judge by "Down in the Chapel," the inmate may be on to something. It is hard to tell whether Mr. Dubler, in his book, is arguing anything in particular about religion in America or religion in prison or the prison system generally. But there is value simply in the details on offer.

Despite his own secularism, Mr. Dubler tries to be fair, and he certainly doesn't portray religion in prison as some sort of opiate. The men who are really passive and tuned out, as more than one inmate observes, are the ones watching television all day, not the ones in Bible study. But ultimately Mr. Dubler is still a liberal academic, and his reason for avoiding the Marxist understanding of faith appears to be more strategic than principled. "Religion as mass delusion doesn't strike me as a particularly interesting story to tell," he writes. Luckily, "Down in the Chapel" is.

Ms. Riley is the author of " 'Til Faith Do Us Part: How Interfaith Marriage Is Transforming America."

A version of this article appeared September 16, 2013, on page A17 in the U.S. edition of The Wall Street Journal, with the headline: Spirit in A Cage.