In June 1938, after his attempts at diplomacy with Nazi Germany and Fascist Italy had failed, Pope Pius XI ordered an American Jesuit, Father John LaFarge, to compose an encyclical denouncing racism and anti-Semitism. The result was a draft called Humani Generis Unitas (The Unity of the Human Race), which LaFarge produced with the help of two other priests. But after Pope Pius XI died early in 1939, his successor, Pius XII, stood by in silence as Nazi Germany began to carry out its Final Solution. The unpublished encyclical was buried in a secret archive, its three authors bound by a vow of silence. For decades Vatican scholars either minimized the importance of Humani Generis Unitas or questioned the document's very existence - until Thomas Breslin, a Jesuit seminarian, uncovered the manuscript in the late sixties. In a disturbing tale of archival intrigue and historical investigation, Georges Passelecq, a Belgian monk, and Bernard Suchecky, a Jewish historian, describe their quest to recover the draft of the encyclical. Undaunted by interminable delays - evasions that only deepened suspicions of Church complicity thirty years after Vatican II - the authors steadfastly pursued their inquiries. Here, published for the first time in English, is the document the Vatican kept hidden for half a century. By examining the circumstances of its creation and the consequences of its suppression, The Hidden Encyclical of Pius XI casts new light on the relations between the Vatican, state-sponsored anti-Semitism, and the Jews during World War II.