Discovery of the password

Robert Langdon and Ambra Vidal, having fled the Guggenheim Museum after Edmond Kirsch's assassination, travel to Barcelona and descend into the crypt of Sagrada Família, where they discover that Kirsch's password is the final line of William Blake's prophetic poem The Four Zoas, cleverly encoded with an ampersand ligature to reach exactly forty-seven characters. The discovery is the culmination of a night-long hunt for the key that will unlock Kirsch's scientific presentation to the world, a hunt that has taken them from the museum to a water taxi, a private jet, Kirsch's penthouse in Casa Milà, and finally to the subterranean chamber beneath Gaudí's basilica.

The Clue in the Crypt

Father Joaquim Beña, the presiding clergyman of Sagrada Família, leads Langdon and Vidal down a winding staircase into the crypt, a vast circular mausoleum where Antoni Gaudí himself is buried. There, in a dimly lit display case to the right of Gaudí's tomb, sits a massive bound edition of The Complete Works of William Blake, propped open to page 163. Beña explains that Kirsch had made a huge donation to the church in exchange for displaying the book, claiming it was a tribute to his late mother, who had admired both Gaudí and Blake. The page in question is not a poem but Blake's famous watercolor etching The Ancient of Days, depicting the god Urizen measuring the heavens with a geometer's compass. Langdon initially wonders if the image is a slap at Christianity, but Vidal realizes the true clue: the book is open to a two-page spread, and the facing page—page 162—contains the handwritten text of Blake's epic poem The Four Zoas.

The Prophetic Line

Vidal crouches down and, using a lantern borrowed from Gaudí's tomb, reads the final lines of the poem aloud: "And Man walks forth from midst of the fires, the evil is all consum'd." Langdon, drawing on his eidetic memory of a Princeton lecture on Blake, recalls that the poem's concluding line is a prophecy of a future without corrupt religion. He directs Vidal to the very end of the text, where she reads: "The dark religions are departed & sweet science reigns." The line is a perfect synopsis of Kirsch's own presentation earlier that night, in which he declared that "the age of religion is drawing to a close, and the age of science is dawning." Vidal counts the letters and finds only forty-six, not the forty-seven Kirsch specified. Langdon then spots the ampersand—the symbol "&"—and recalls his own symbology lecture on the ampersand's origin as a ligature of the Latin letters E and T. By substituting the Latin "et" for the ampersand, the line becomes exactly forty-seven characters: "thedarkreligionsaredepartedetsweetsciencereigns." The code within a code is cracked.

The Password Confirmed

With the password in hand, Langdon and Vidal race back up the crypt stairs, only to find that the two Guardia Real agents guarding the sanctuary have been killed by Admiral Luis Ávila, who has tracked them to the church. In the ensuing confrontation on the spiral staircase, Langdon manages to throw Ávila to his death, but not before sustaining a head injury. When he regains consciousness, Vidal reminds him of the password and asks where to find Winston, Kirsch's synthetic intelligence. Langdon, having deduced that Winston's self-portrait at the Guggenheim—a Miró-style painting with a single eye—is actually a map of Barcelona, directs the helicopter to the Barcelona Supercomputing Center, housed in a decommissioned church. There, he enters the password on Kirsch's childhood Tandy TRS-80 computer, and the presentation is launched to a global audience of over two hundred million people.