Science admits no dilution. If even trace amounts of a foreign element—politics, faith, prejudices, personal convenience—are added to its batch of sweet reason, science may do artful, surprising, even inspirational things. But it is no longer science.

Despite—or maybe because of—this hard truth, those men charged by the Roman Catholic Church with investigating claims of the miraculous deserve special regard. From the liquefaction of the blood of San Gennaro to the apparitions of Our Lady of Scottsdale, Ariz., church-appointed investigators vet each purported miracle in a spirit of caution and rational inquiry. But a scientific approach to the miraculous is ultimately as futile as St. Augustine's effort to comprehend the Trinity. This absurd dilemma is what gives Randall Sullivan's The Miracle Detective its quirky charm.

Prompted by reports of a 1994 vision of the Virgin Mary in Boardman, Ore., Sullivan set out to profile the church's supernatural investigators. This group includes some formidable figures, including the sharp, literarily astute Dominican Father Gabriel O'Donnell, and the celebrated Capuchin Father Benedict Groeschel, a star of the EWTN channel, an author of a celebrated study of mysticism and a dead ringer for R. Crumb's Mr. Natural.

Both men play very minor roles here, however, and The Miracle Detective is emphatically not the book Sullivan set out to write: As he encounters blissed-out visionaries and has a few supernatural episodes of his own, Sullivan places his own search for religious truth front and center. Most of the book concerns the author's experiences in Medjugorje, the village in Bosnia-Herzegovina where someone claiming to be Mary has been appearing to six ethnic Croatians since 1981. Sullivan catalogues his doubts and ably presents the views of skeptics (among them the local bishop), but these counterarguments get generally short shrift (the growing skepticism of a priest who began as a fervent Medjugorje supporter strikes the author as "like the progress of a disease"), and we find frequent instances in which this or that formerly skeptical scientist concludes that some phenomenon "cannot be explained naturally, and thus can be only preternatural or supernatural." The book is an extended argument for the authenticity of the Medjugorje apparitions.

Sullivan, a contributing editor of Rolling Stone, retains a sharp journalistic instinct, and his exhaustive rendering of the Medjugorje story is, if not the best journalistic account of the subject (the Medjugorje literature is too vast to allow a confident claim), certainly the best one I have read. Though he is too dismissive of the political dimensions of Mariology, and in particular of Marian anti-communism (particularly in light of the way Franjo Tudjman used the Medjugorje apparitions in delivering his death blow to the Yugoslav federation), Sullivan brilliantly situates the apparitions within the context of the Balkan war. Amazingly, this makes him something of a pioneer: You could read whole volumes of dopey Medjugorje witnessing without suspecting that the Balkans endured a vicious civil war in the 1990s.

The Miracle Detective contains vivid passages, nicely rendered theological history and suspenseful incidents. With so much to recommend in Sullivan's book, I hate to carp, but the line editing is often shoddy (John Cornwell, not Cornwall, is the author of Hitler's Pope), Sullivan's credulity is frequently stunning (in reference to the Serbs of the 1920s, he uncritically recounts the ancient saw about soldiers impaling babies on their swords—a charge leveled at every foe from the Apaches to the Iraqis), and his gaffes are sometimes mortifying (I wanted to put the book down when squadrons of B-52s, an aircraft not even introduced until 1951, were observed bombing Germany during World War II).

I mention such picayune stuff because I agree with the Scottsdale seer who at one point advises the author: "You should pray for the gift of discernment, Randall, because you need it tremendously." Case in point: Early in the book, the Boardman seer, in a history of the dispirited secular wanderings she endured before her vision (tales of pre-conversion dissoluteness are pro forma among Marian prophets), tells Sullivan: "Going through high school, I was taught about evolution, that we were monkeys, and there was really no God." Public high school teachers are among the most risk-averse people in America, and the idea that one would risk a definitive statement on God's nonexistence is laughable; the bit about the monkeys is a typical ill-informed caricature of Darwinism. These comments are fibs and should send up a red flag about the speaker's honesty on larger matters, but Sullivan takes no notice.

He could have learned a few things from O'Donnell, Groeschel and some of the other hardheaded priests he encountered. Self-doubt and second thoughts provide dramatic reading, but they are no substitutes for rational skepticism. And without that, Mariology doesn't even make good pseudo-science.