An ornate $69 million, 17-story building at the corner of 34th Street and Lexington Avenue is the national headquarters of Opus Dei, an international Roman Catholic prelature whose secrecy, financial holdings, and connections with conservative leaders has given it a dark reputation. But it’s safe to say Opus Dei had an image problem even before Dan Brown’s mega-selling “Da Vinci Code” included as a villain an albino Opus Dei monk who penitently whips himself bloody and murders innocent people as part of a global conspiracy. Observers inside and outside the church express concern about the group’s recruitment tactics, treatment of women, and corporal punishment practices.

Opus Dei was founded in Spain in 1928 by an Aragonese priest, Josemaria Escriva. Now Saint Josemaria – canonized just 27 years after his death in 1975. The man knew how to draw strong reactions. For opponents, he was intemperate, complicit with Spanish dictator Francisco Franco, and theologically radical. For supporters, he was a visionary who reinvigorated spirituality among lay Catholics.

Opus Dei today has 85,000 members divided into various classes. Some 70% are supernumeraries – people with families who have normal jobs; 20% are celibate numeraries who live in Opus Dei centers; the rest are numerary assistants (female domestic caretakers), associates (celibates who don’t live in Opus Dei centers), and priests.

John Allen Jr. is the longtime Vatican correspondent of the National Catholic Reporter. Using unprecedented access to Opus Dei officials and members, he meticulously describes the organization and dispels some of the outlandish claims against it in his new book (Doubleday Religion, 416 pages, $24.95). The secrecy, for instance, is a byproduct of a central teaching of Opus Dei: Lay members see broadcasting their membership in Opus Dei as a violation of the humility under which they should operate. They can serve God just as well as priests do by performing their secular work to the best of their ability, but humility in their work is key.

Mr. Allen diligently and patiently explains the conservatism (Opus Dei members are theologically traditional more than politically conservative), treatment of women (only women are allowed to serve in the domestic class but women do have leadership positions within the group), and corporal mortification (only done by numeraries and not as bad as it sounds). In the process he paints a sympathetic portrait of a largely misunderstood prelature of the Roman Catholic Church.

Opus Dei (literally “the work of God”) places a great deal of emphasis on the sanctifying nature of work. All across the globe, Mr. Allen sees this central tenet in action. In Peru he speaks with a laundromat worker and supernumerary who says he doesn’t clean shirts just for clients, but for God. Mr. Allen quips, “Quite an advertisement, among other things, for laundries staffed by Opus Dei members.”

Mr. Allen traipses through anecdotes of virtuous and kind Opus Dei members whose lives were transformed by Escriva’s teachings but never adequately explains why Opus Dei is controversial to begin with. If Opus Dei is to be judged by the lives of its followers, one would think Robert Hanssen, a supernumerary and FBI agent, might receive more ink. He spent 15 years passing deadly secrets to Russians in exchange for $1.4 million in cash and diamonds, betrayed his wife by keeping a stripper, and generally led a sex-obsessed double life.

Mr. Allen’s method is to describe a criticism of Opus Dei and then rebut the charge, with varying success. Do high-level Roman Catholics in many countries worry about Opus Dei’s influence on the Vatican? They shouldn’t, since Opus Dei’s physical presence there is limited. Do parents tell horror stories about children leaving to join the “Work”? Yes, but “[r]esentment from parents about religious vocation is hardly limited to Opus Dei.”

As lopsided as the investigation is, Mr. Allen’s defenses are tolerable and informative, until he explains why Opus Dei keeps some of its publications secret. Internal magazines are hidden from the public because they contain personal anecdotes and experiences. What church publication doesn’t? When opponents charge that journals are republished – Soviet-style – to remove pictures of ex-members, Mr. Allen argues that former members are not always scrubbed from the pages.

Likewise, the Opus Dei catechism has to be held under lock and key because it’s not a self-standing document but a launching point for discussion. “Absent that context, it could be misunderstood.” Now, really: All religious texts depend on context but very few depend on secrecy.

At a time when rumormongers are running wild with Opus Dei conspiracies, a sane defense of the group is in order. But Mr. Allen keeps a journalistic distance from the real action – the theological divide between Opus Dei and its liberal and conservative critics. Escriva’s vision and mission are theologically radical. Laypeople supervising priests is arguably anti-clerical. The view that all Christians can co-redeem the world could be perceived as denigrating the work of Jesus Christ. Teaching that day jobs are the spiritual equivalent of the Eucharist – both are “places where Christ is made present” and sacrifices are given to God – might be anti-sacramental.

Mr. Allen’s book fails to adequately deal with these issues or arrive at an answer to the most obvious question about Opus Dei. Is there any meaningful reason why Opus Dei is so widely mistrusted?

Ms. Ziegler is a Phillips Foundation journalism fellow in Washington. She is writing a book on American civil religion.