Between 1200 and 1210 the German writer Wolfram von Eschenbach composed a romantic poem called Parzival. Tens of thousands of lines long, it drew on the legends of King Arthur, which had been wildly popular across Europe for decades. These stories delighted aristocratic audiences with tales of love, chivalry, questing, betrayal, magic and combat.

Earlier writers such as Geoffrey of Monmouth and Chrétien de Troyes had already established the world Eschenbach described and provided many of the plots. So Eschenbach was essentially updating, expanding the characters’ adventures and giving the tales a flavor which he thought his own readers would enjoy. One of the ingredients he added was the appearance of a military order called the Templeise, warriors sworn to chastity who help the Fisher King keep watch in their “temple” over the Holy Grail.

The real Templars were still very much alive and kicking in 1200. But the fact that writers were beginning to fictionalize them wasn’t surprising. By the first decade of the 13th century the order was well known. Anyone with a passing interest in the Holy Land could not fail to have heard of the Templars’ deeds.

They had clashed with Saladin, starred in Richard the Lionheart’s crusade and manned dozens of castles in the Holy Land. They had become bound up in the Reconquista, established a presence at most of Europe’s royal courts, acquired estates all over Christendom and made powerful enemies.

It was not much of a jump to become a literary trope as well.

Even in their own day the Templars were of vastly more interest to writers of fiction than the Hospitallers and Teutonic Order. The Templars’ rivals long outlived them, but left nothing close to the same impression in the popular imagination. Only the Templars can really be said to have passed from the realm of reality into mythology.

To be fair, the Templars were different from the other major international military orders. Uniquely, from the beginning they were knights who took up a religious calling, rather than servants of a hospital that added a paramilitary wing. This gave them a certain quality that was useful for medieval romance: They corresponded exactly to the archetype of the truly chivalrous men—violent but chaste, tough but pure of heart, merciless but godly. They were the ideal toward which all knights in Arthurian legend strove.

But this is only part of the story, and much of the Templars’ enduring popularity must also be attributed to the manner of their fall. Half a century after James of Molay’s death, the Florentine poet and storyteller Giovanni Boccaccio composed a book known as De Casibus Vivorum Illustrium, a compilation of great men’s lives, selected to illustrate the turning of fortune’s wheel.

A beautiful illuminated French edition of Boccaccio’s book, completed around 1409, contains a vivid (if historically questionable) image of James of Molay being burned at the stake along with three colleagues, in front of a satisifed Philip IV. It also contains a detailed account of James’s life. In this telling, his death and the order’s downfall were the result of divine vengeance. The order’s wealth and status grew in inverse proportion to its religiosity until its members received the ultimate punishment for their sins. That was quite a glossing of the story, but it had poetic shape and a healthy dose of natural justice. It was an appealingly simple narrative, in which the Templars’ collective fate pivoted on the moral flaws of greed and pride.

In the popular consciousness the Templar story has tended toward this trajectory ever since. From Boccaccio to Sir Walter Scott to Ridley Scott, generations of storytellers have found rich material in the gap between the Templar ideal and real life. Sir Walter Scott created the thuggish, lascivious and power-crazed Templar villain Sir Brian de Bois-Guilbert in his 1820 novel Ivanhoe, while Ridley Scott depicted the real-life Templar master Gerard of Ridefort as a brutalized villain in his 2005 film “Kingdom of Heaven.”

Their legacy today is not always benign. A notorious drug cartel founded in Western Mexico in 2011 named itself Los Caballeros Templarios: The Knights Templar. Its members have been held responsible for a range of violent crime associated with the drug business, including murder, trafficking and extortion, but they have attempted to attach a higher purpose to their activities, blending a stern Christian zealotry with populist left-wing politics. The group even produced a 22-page booklet called the “Code of the Knights Templar,” inspired by the medieval Rule. “The Templar should be a model of gentlemanliness,” it reads, exhorting its followers to avoid “brutality, drunkenness in an offensive manner, immorality, cowardice, lying and having malicious intentions,” as well as “kidnapping for money” and using drugs “or any mind-altering substance.” It also includes frequent reminders that errors and disrespect towards the organization and fellow members will invite summary execution.

The popular pseudohistory Holy Blood, Holy Grail, first published in 1983, suggested and popularized the idea that the Templars were linked to a corporation known as the Priory of Sion, established to guard a secret bloodline of kings descended from Jesus Christ and Mary Magdalene. Dan Brown’s bestselling The Da Vinci Code, published in 2003 and subsequently made into a successful movie, presented broadly similar ideas faux-seriously, adding greatly to the novel’s success but leaving readers to work out for themselves whether or not the author’s hypotheses had some basis in fact. (Many concluded that they did.)

In Umberto Eco’s novel Foucault’s Pendulum, published in 1988 to a similarly misplaced credulity, three writers concoct a method for knitting together all of world history in one giant conspiracy, which they call the Plan. This features secret cells of Templars waiting to see revenge for their destruction by the king of France.

Although archly postmodern, obviously satirical and directly mocking of those who place the Templars at the heart of a grand scheme for world domination, Eco’s novel has added to the popular mystique about the order.

What if just 10 percent of it were true?