When Remy Cordonnier, a French librarian, realized the truth about the tome before him, he couldn’t suppress a swell of excitement. “I was trembling when I picked it up and realized what it might be,” he would later tell the Telegraph.

His emotion was warranted: It was a First Folio. Called “the most important work in the English language,” First Folios include much of William Shakespeare’s work — 36 plays — and introduced the world to “Macbeth.” Published seven years after Shakespeare’s death in 1623, they are credited with keeping Shakespeare’s legacy alive and, with only 223 of them known to exist today, they are  among the world’s most valuable books.  They’ve sold for up to $6 million, and the quest to find them over the years has lured a Shakespearean cast of characters, including mercurial billionaires, Tokyo street gangs and serial con men.

And now, in Cordonnier’s hands, was another First Folio. But could he be sure? It had languished in obscurity in a public library in Calais, France, for centuries. So he called a man named Eric Rasmussen, an American Shakespeare expert who happened to be in London and could authenticate the folio. Rasmussen caught a Eurostar train to the library last weekend. Within minutes, he knew what was before him, cradled in red velvet: an authentic First Folio.

The discovery of the book, first reported by the French newspaper La Voix Du Nord,  “is huge,” Rasmussen told the New York Times. “First folios don’t turn up very often, and when they do, it’s usually a really chewed up, uninteresting copy. But this one is magnificent.” The book, originally believed to contain 300 pages, is missing about 30, including the title page and a portrait of Shakespeare — a distinguishing mark of First Folios.

And so, centuries ago, the public library at Saint-Omer in the north of France incorrectly cataloged the folio as an 18th century book, rather than an original from the 1600s.

Cordonnier learned the truth only by chance during a recent search for valuable English texts in preparation for an upcoming exhibit. “It had been wrongly identified in our catalogue as a book of Shakespeare plays most likely dating from the 18th century,” Cordonnier told the Guardian. “I didn’t instantly recognize it as a book of value. It was heavily used and was damaged. It had seen better days.”

But this was nonetheless huge.

“It was very emotional to realize we had a copy of one of the most famous books in the world,” Cordonnier told the Times. “I was already imagining the reaction it would cause.”

Each First Folio is different. Each tells historians, who can discern much in textual variants, something new about Shakespeare. This one may inform more than most — and rekindled controversial questions regarding the greatest writer the English language has known: Was Shakespeare, thought to be Protestant, secretly Catholic?

“What is really interesting,” Rasmussen told Agence France-Presse, “is that it clearly came from the college of Jesuits in Saint-Omer, founded in the late 16th century during Queen Elizabeth’s reign when it was illegal for Catholics to go to college.”

The folio was also inscribed on the first surviving page, Rasmussen told the Times, with the name “Neville,” an alias for a prominent English Catholic who attended the Jesuit college. “People have been making some vague arguments” about Shakespeare being Catholic, he told the Times, “but now for the first time we have a connection between the Jesuit college network and Shakespeare. … The links become a little more substantial when you have this paper trail.”

Even before the discovery of the book, many Catholics and the Vatican suspected Shakespeare was secretly Catholic — but who could know? Both Anglicans and Catholics claimed him as their own. There’s also a dispute about whether he was buried in a Catholic or Protestant church.

“There were many Catholics among [Shakespeare’s] family, friends, and neighbors, all of whom suffered under crippling new laws,” wrote author Clare Asquith in 2005. “The current consensus is that his childhood was deeply imbued with the ‘old religion,’ and that as an adolescent he may have been involved in the 1580 Jesuit mission led by the charismatic Edmund Campion.”

Then there are his writings, in which some academics discerned signs of latent Catholicism. In “Hamlet,” he referenced Purgatory, which prompted Vatican City newspaper L’Osservatore to declare the playwright “convincingly adhered to the Catholic faith.”

While that is far from clear, this recent connection to Catholicism has indeed revived the debate. As one French expert told the Times, the new Folio “could be part of the puzzle of Shakespeare’s place in Catholic culture.”