Gerard DeGroot is a professor of history at the University of St Andrews in Scotland. His latest book is “Back in Blighty: The British at Home in World War One.”

The True Story of a Convent in Scandal

By Hubert Wolf

Translated from the German by Ruth Martin

Knopf. 476 pp. $30

Sister Maria Luisa was the novice mistress at the Sant’Ambrogio convent in Rome. She was intelligent, charismatic and stunningly beautiful. She was also a sociopath, embezzler, false saint, sexual predator, pathological liar and murderer.

“The Nuns of Sant’Ambrogio: The True Story of a Convent in Scandal” by Hubert Wolf. (Knopf/ )

How do you solve a problem like Maria?

Had her crimes been committed outside the convent, the problem would have been easily solved. Maria Luisa would have been arrested, tried and quickly executed. But she was a nun, which meant that the real concern was not the victims she raped and murdered, but the threat she posed to the Catholic Church.

The jacket of “The Nuns of Sant’Ambrogio” promises an “incredible story of how one woman was able to perpetrate deception, heresy, seduction and murder in the heart of the Church itself.” Given that jackets are designed to sell books, the hyperbole is entirely understandable. In fact, however, this astonishing book is much more than a true-crime thriller about murderous lesbian nuns. It’s also a very serious study of how the church deals with scandal. As Hubert Wolf points out, we aren’t supposed to know about this story.

The scandal begins with Maria Agnese Firrao, a manipulative tyrant who founded the convent in 1806. She was a seriously disturbed woman who used lesbian initiation rites to cast a spell over novice nuns. Bleeding stigmata on her hands, feet and breasts were supposedly evidence of her saintliness. In addition, she often wore an iron mask pierced with nails and once trapped her tongue under a heavy stone, presumably to demonstrate that she was incapable of blasphemy or falsehood. When the Vatican, alarmed by her “false holiness,” investigated, evidence emerged of “lasciviousness and carnality with demons.” That was church-speak for a long affair with a priest, which had resulted in two abortions.

Firrao was subsequently removed and sent to a cloistered convent in Gubbio. Such was her power, however, that she continued to exert control over Sant’Ambrogio through the deep reverence she had cultivated. As a result, the ethos she had carefully established continued long after her departure. That explains why Maria Luisa was subjected to the same initiation rites soon after she entered the convent at age 13. Persuaded to perform oral sex on the new Abbess Maddalena, she was told “that I should have no fear: this was not a sin, but a holy thing and a gift from God.”

The ambitious Maria Luisa rose quickly within the convent. Very soon, the abused became an abuser more catholic in her criminality than her predecessors. She used her beauty and charm to dominate those around her, convincing them that she had a direct line to heaven. Forged letters purportedly from the Virgin Mary directed both nuns and priests to have sex with Maria Luisa, for purposes of purification. Ornate rings, supposedly gifts from God, were bought with money embezzled from the convent’s legacy. Three nuns who refused to bend to her will were murdered, their deaths presented as evidence of God’s desire to cleanse the convent.

Maria Luisa’s tyrannical control might have continued undetected if not for the arrival in September 1858 of Princess Katharina von Hohenzollern-Sigmaringen, a devout German aristocrat. Though at first delighted to escape her shallow patrician luxury, Katharina soon discovered that the convent was a den of vice. Matters reached a tipping point when she was asked to translate a letter from a certain Pietro the Americano, which turned out to be a catalogue of his perverted fantasies. Immune to Maria Luisa’s charms, Katharina revolted.

Maria Luisa, coveting Katharina’s immense wealth, could not afford to let the princess become a whistleblower. She therefore had to die — quickly. Over the course of a week, Katharina was fed opium, belladonna, alum, turpentine and various other poisons. Much to Maria Luisa’s dismay, however, she refused to die. She eventually managed to escape and, through her aristocratic connections, made allegations of false holiness, sexual perversion and murder within the convent. The church was at first inclined to dismiss Katharina as a lunatic, but a lengthy investigation and trial eventually revealed that the scandal went much deeper than even the princess understood.

Wolf, a professor of ecclesiastical history at the University of Münster in Germany, reconstructs this incredible story from Inquisition transcripts remarkably lurid in their detail. A less-principled author would have turned this into a tale of naughty nuns and perverted priests, given it a racy cover, and watched the royalties roll in. Wolf is, however, a scholar of immense integrity who realizes that this story does not require embellishment. He’s also keen to show what Sant’Ambrogio reveals about how the Vatican deals with scandal. In this case, senior church figures who had “venerated [Maria Luisa] as a living saint, now dropped her like a hot potato. . . . Their claims chimed with contemporary clichés about women: what else could one expect from the weaker sex, the daughters of Eve, who were so easily seduced by evil and then seduced men in turn?”

The Vatican investigation was brutally thorough — but even more so was the subsequent coverup. This case was never supposed to see the light of day. Wolf found the Sant’Ambrogio records only because they had apparently been misfiled. That raises an interesting question relevant to the church of today: Are there similar scandals that have been more carefully buried? Satisfied with telling this one perfect story, Wolf doesn’t speculate. Yet the conspiracy he describes does suggest that the Vatican is disturbingly proficient at solving problems like Maria.