A fanatic, as the saying goes, only does what God himself would do if he knew all the facts of the case. This absolute certainty, an "unswerving confidence in the rightness of one's cause," is what Cullen Murphy describes as "the inquisitorial impulse" in his informative, wide-ranging, and surprisingly -- given the grim subject -- entertaining "God's Jury."

Murphy, an editor at "Vanity Fair" and author of "Are We Rome?," a 2007 book that drew insightful parallels between the United States and the Roman Empire, examines the Inquisition here. The term normally refers to the Catholic Church's official fight against heresy, a campaign that commenced in the 13th century and endured into the 20th.

For Murphy, this is just the departure point for a wide-ranging meditation on the relationship between, as his subtitle puts it, "the Inquisition and the making of the modern world."

This is, of course, a major paradox. After all, the Inquisition's "chief target was modernity itself." Nevertheless, Murphy persuasively argues that we should view the Inquisition not as merely some medieval "relic but as a harbinger" of modern secular states' ability and willingness to surveil, censor and punish those who threaten the established order.

It's actually more apt to speak of multiple inquisitions. The term "the Inquisition" is a convenient shorthand for medieval Spanish, Portuguese and Roman variants. Murphy briefly traces each, lingering over greatest hits such as Grand Inquisitor Tomas de Torquemada's cruelty and the persecution of Galileo. He depicts the Inquisition as "a modernizing institution" that developed sophisticated bureaucratic procedures -- codified standards, meticulous record keeping -- to accomplish the vital work of rooting out heretics.

Murphy makes arresting connections between medieval guidebooks for conducting inquisitions and the techniques employed by modern military, intelligence and police interrogators. Even the euphemisms are similar. The Inquisition employed rigoroso esamine -- "rigorous examination" -- while the U.S. has recently subjected suspected terrorists to "enhanced interrogation." Both, of course, stand for "torture."

One of this book's most sobering sections invokes the Canadian intellectual Michael Ignatieff's depiction of torture chambers as "intensely moral places." Murphy explains that those who torture in the name of God or the state don't avoid moral thinking; rather "they override the obvious immorality . . . by the presumptive morality of the larger endeavor." Paraphrasing Dostoyevsky, with God everything is permitted. The ends justify the means.

Still, despite its chilling task, the Inquisition exhibited its share of the same "bungling and administrative idiocy . . . one finds in 'Dilbert' or 'The Office.' " Murphy quotes one scholar's description of a 15th-century Inquisition manual as " 'a strange amalgam of Monty Python and 'Mein Kampf.' "

Censorship was "often myopic, childish, capricious, or daft." Indeed, its most insidious consequence may have been in books never-written due to self-censorship. Impishly, Murphy also observes that placing titles on the Index of Forbidden Books could serve primarily to tempt curious Catholic lads, if not always reliably, "as those who have tried reading Hobbes or Pascal under the covers with a flashlight will have discovered."

Murphy deploys his own Catholic bona fides early on; he's definitely more Dorothy Day than Opus Dei. Still, one suspects that some readers will resent the barbs he hurls at Holy Mother Church. A few are merely irreverent: jibes at the "gerontocrats" running the Vatican or the well-worn "panzerkardinal" to refer to the current, German-born Pope. Others are more serious: The Church has failed to square up to its misdeeds prosecuting the Inquisition.

Similarly, Murphy's comparisons between the Inquisition and detainee abuse at Guantanamo Bay are well taken. So are the parallels to the witless attempts to proscribe the menace of Sharia in Tennessee. Yet some will wonder why Murphy doesn't get just as exercised over the far nastier abuses in enforcing orthodoxy in Cuba or in those unhappy regions under the Taliban's sway.

Part historical survey, part "Da Vinci Code," "God's Jury" is also a subtle, learned warning against intolerance in our own time. Murphy cites an earlier Inquisition historian in what could serve as the motto for his own book. "I have not paused to moralize, but I have missed my aim if the events narrated are not so presented as to teach the appropriate lesson."

Alan Cate, a retired altar boy, teaches history at University School in Hunting Valley, Ohio.