You’ve got to hand it to Bloomsbury, the British publishing house. It knows how to market books about dark secrets in rambling old castles with long corridors. It made millions from Hogwarts with bad guys Voldemort and the Death Eaters battling good guys Harry Potter and his friends.

And it stands to make millions from its latest blockbuster about bad guys in the Vatican battling ... well, there are no good guys in Bloomsbury’s Vatican.  

In the US, the UK and Canada the title of the book is In the Closet of the Vatican. Elsewhere, leaving little to the imagination, it is called Sodoma. The author, Frédéric Martel, is French, but the book will be released simultaneously in eight languages on February 21.

And it is on February 21 that the Vatican opens a world-wide summit of bishops to study clerical sexual abuse. Bloomsbury will be surfing a monster wave of prurient interest in the hidden sins of Catholic priests. This is sheer PR genius. Ka-ching! Ka-ching!

The publisher fired an opening salvo in its publicity campaign a couple of days ago with a New York Times op-ed by Frank Bruni, a gay columnist. Given his sympathy for LGBT causes and his hostility toward the Catholic Church, it is remarkably even-handed. He says that some language in the book “is at once profoundly silly and deeply offensive” and that the sourcing of many of the book’s lurid claims is “vague”.

And he reports that the book’s most eye-popping factoid, the one which figures in all the headlines drip-fed from the Bloomsbury PR machine, that 80 percent of the male clerics who work in the Vatican are gay, is “neither knowable nor credible”.

Martel, who is openly homosexual, seems to have carved out a niche for himself -- fat, fact-filled books about gay culture. In 1996 he published The Pink and the Black, Homosexuals in France since 1968 and in 2012 Global Gay: How gay culture is changing the world. He says that he conducted 1,500 interviews in the course of writing his current book – which is 500 more than the number of people who actually live in the Vatican. If that’s true, it’s not necessarily a good sign. As the old computer maxim goes, garbage in, garbage out. It depends on how honest and credible his sources are.

To judge from an interview with the source responsible for the 80 percent figure, credibility may be an issue. Francesco Lepore’s story opens the book and he is apparently Martel’s key informant. A priest working as a Latin scholar in the Vatican, he eventually walked off the job to live as a homosexual. He was fed up with living a double life. But he wrote in an Italian newspaper that there are “some inaccuracies” in how Martel has reported his experiences. We’ll have to wait for publication to see just how credible his allegations are.

Extraordinary claims require extraordinary evidence. I’ll be interested to see how much of the book is anonymous tittle-tattle and gossip and how much it relies on embittered informants with an axe to grind. In the pre-publication publicity, the name of Cardinal Alfonso López Trujillo, of Colombia, has figured prominently. Martel describes him as the worst of hypocrites, a cleric who damned homosexuals by day and consorted with rent boys by night. Conveniently for the author, López Trujillo died a decade ago so it will be almost impossible to check these allegations.

It seems, too, that some of Martel’s evidence is based on the assumption that the more rigid and homophobic clerics are, the more likely they are to be closeted homosexuals. Therefore, he suspects that three of the four conservative Cardinals who signed the “Dubia” as a protest against Pope Francis were secretly gay and at least a quarter of the Cardinals on the Congregation for the Doctrine of the Faith. In fact, by this logic, anyone who supports the Catholic Church’s teachings on homosexuality must be a latent homosexual. Yes, you guessed it, Benedict XVI is one of these, according to Martel. 

It must be admitted that persistent rumours, ex-Cardinal McCarrick, occasional scandals in Rome, and Pope Francis’s fulminations about hypocrisy suggest that there are a number of gay clerics in and around the Vatican. But Martel’s figure of 80 percent is simply ludicrous. It’s an insult to the hard-working men (and women) who have dedicated their lives to serving the Church.

When the book is released I hope that reviewers stress-test every preposterous allegation. I’m sure that most of them will collapse.

Michael Cook is editor of MercatorNet.