In Ian Caldwell’s follow-up to his international sensation The Rule of Four, a lost gospel, a contentious relic, and a dying pope’s final wish converge to send two brothers—both Vatican priests—on an intellectual quest to untangle Christianity’s greatest historical mystery.

Ten years ago, Ian Caldwell and Dustin Thomason’s The Rule of Four became a literary phenomenon that earned comparisons to Umberto Eco’s The Name of the Rose and Donna Tartt’s The Secret History. Hailed as “ingenious…profoundly erudite” (The New York Times), “compulsively readable” (People), and “an exceptional piece of scholarship” (San Francisco Chronicle), it spent forty-nine weeks on the New York Times bestseller list, sold nearly two million copies in North America, and was translated into thirty-five languages around the world. Now, after a decade of painstaking primary research, Ian Caldwell returns with a new intellectual thriller that confirms his place among the most ambitious popular storytellers working today.