When Franz Kafka invented the ordeal of Joseph K. in “The Trial” – the hidden charges and labyrinthine procedures, the faceless judges and impotent defenses – he seemed to be writing a parable for the 20th century. But after reading “The Spanish Inquisition” (Yale University Press, 256 pages, $26), the brief new study by the eminent Spanish historian Joseph Perez, you could be forgiven for thinking that Kafka was not inventing a fable, but simply writing an accurate history of the Inquisition.

Mr. Perez’s book, which first appeared in France in 2002, concentrates on the institutions and procedures of the Inquisition, with only a few glances at its religious, ideological, and political purposes and effects. This narrow focus succeeds in bringing home to the reader what is most horribly familiar about the Inquisition: Its perversion of the forms of law, which has become a standard feature of modern police states and is the source of the nightmarishness of Kafka’s novel.

Indeed, many of Mr. Perez’s examples could be taken almost word for word from “The Trial.” Joseph K. is warned that “the court records, and above all the writ of indictment, are not available to the accused and his defense lawyers.” In his chapter on “The Trial” – and surely that title is no coincidence – Mr. Perez explains what happened to a defendant hailed before the Inquisition: “Giving no details, [the judges] invited the accused to tell them why he had been arrested and to make a full confession.”

“Invited the accused to tell them why he had been arrested” – the ingenious perversity of this method, which both prevented the victim from mounting a defense and encouraged him to incriminate himself further, extended to every aspect of the trial. “He could reject certain witnesses,” Mr. Perez writes, “but as he did not know who they were, he had to list the names of all those he suspected of wishing to harm him; then, if any of the witnesses did appear on the [prosecutors’] list, they were automatically rejected.”

The trial itself depended heavily on torture – including the “ordeal by water,” very similar to the “waterboarding” practiced by American interrogators in Iraq – and it could go on for many years. (No less a figure than Archbishop Carranza of Toledo was incarcerated for 17 years before a verdict was pronounced.) The most Kafkaesque feature of the Inquisition, however, was its concern with the mental state of the accused. The inquisitors stated explicitly what Joseph K. only gradually comes to suspect: that the real purpose of the proceedings was not to prove a defendant’s crimes, but to convince the defendant himself of his own guilt.

The idea of a court devoted to scrutinizing the orthodoxy of Roman Catholic believers was not exactly new when the Spanish Inquisition was established in 1478. What made the “Holy Office” so novel, and so fearsome, was the way it put religious power in the hands of the secular authorities. The Grand Inquisitor himself – Torquemada was the first and most notorious – was technically appointed by the pope, but he was nominated by the Spanish monarch, as were all his subordinates. This double authority meant that the Inquisition could run roughshod over all the privileges cherished by both nobility and clergy, as well as any laws that stood in its way. It could summon anyone at any time, for almost any reason; the accused had no rights in the process, and the verdict admitted of no appeal.

When the Inquisition was founded, its main purpose was what Mr. Perez calls “the eradication of Semitism.” For seven centuries, Spain had been partly governed by Arab rulers, and its Christians lived alongside a great number of Muslims and Jews. When the Catholic monarchs Ferdinand and Isabella completed the “reconquest” of the Iberian Peninsula, it became urgently important to make Spain more like the rest of Christendom, to replace infidel diversity with pious uniformity. The first targets of the Inquisition, then, were “Judaizers,” converted Jews suspected of practicing the faith of their ancestors. These were the victims of the Inquisition’s bloodiest period, 1480-1500, when the expulsion of the Jews and the persecution of conversos largely succeeded in destroying Jewish life in Spain.

But the final solution to Ferdinand and Isabella’s Jewish problem did not mark the end of the Inquisition. As Mr. Perez shows, its mandate gradually expanded to cover new categories of offenders: Muslims, Lutherans, the antinomian mystics known as Illuminists, witches, bigamists, ordinary Christians accused of blasphemy, and political enemies of the regime. The vigor of the Inquisition was maintained throughout the religious wars of the 16th century. Afterward it gradually declined, until by the end of the 18th century it was largely a moribund bureaucracy, some of whose own leaders were freethinkers. Like the Holy Roman Empire, the Spanish Inquisition was dealt its deathblow by Napoleon; it was officially abolished in 1834.

The real mission of the Inquisition, Mr. Perez makes clear, was not just to punish heretics but to instruct the whole nation in the perils of disobedience. That was why the inquisitors, like Stalin’s NKVD, always sought not just a conviction but a public admission of guilt. “The accused,” as Mr. Perez writes, “must declare himself to be guilty and express his repentance.” If he did so, he could be “reconciled” to the church, suffering only the minor punishments at the inquisitors’s disposal: confiscation of property, exile, flogging, or galley slavery. As Mr. Perez explains, the infamous “auto da fe,” usually thought of as synonymous with burning at the stake, was in fact a public spectacle, usually of great pomp, at which crimes and recantations were read out for the edification of the people. Only the “impenitent,” who stubbornly refused to acknowledge they had done anything wrong, were actually sent to the stake.

Over the three and a half centuries of its life, Mr. Perez estimates, the Inquisition held 125,000 trials, and executed some 10,000 people. (Oddly, the book’s jacket copy claims that “nearly 32,000 people were publicly burned at the stake,” seemingly relying on a 19th-century estimate that Mr. Perez quotes only to refute.) But it played a role in the Western imagination out of proportion to the actual number of its victims, serving as a focus for all kinds of fears about purity and pollution, tyranny and bigotry. Today, long after it has vanished, the Inquisition still deserves study as a reminder of the potential consequences of fusing church and state. “The confusion of the temporal and spiritual spheres,” as Mr. Perez writes, “contained the seed of one of the most dangerous temptations of the modern world: the tendency to make ideology the obligatory complement of politics.”