The Vatican’s silence in the face of Nazi atrocities remains one of the great controversies of our time. History has accused wartime pontiff Pius the Twelfth of complicity in the Holocaust and dubbed him “Hitler’s Pope.” But a key part of the story has remained untold.

Pius ran the world’s largest church, smallest state, and oldest spy service. Saintly but secretive, he skimmed from church charities to pay covert couriers, and surreptitiously tape-recorded his meetings with top Nazis. When he learned of the Holocaust, Pius played his cards close to his chest. He sent birthday cards to Hitler—while secretly plotting to kill him.

Church of Spies documents this cloak and dagger intrigue in shocking detail. Gun-toting Jesuits stole blueprints to Hitler’s homes. A Catholic book publisher flew a sports plane over the Alps with secrets filched from the head of Hitler’s bodyguard. The keeper of the Vatican crypt ran a spy ring that betrayed German war plans and wounded Hitler in a briefcase bombing.

The plotters made history in ways they hardly expected. They inspired European unification, forged a U.S.-Vatican alliance that spanned the Cold War, and challenged Church teachings on Jews. Yet Pius’ secret war muted his public response to Nazi crimes. Fearing that overt protest would impede his covert actions, he never spoke the “fiery words” he wanted.

Told with heart-pounding suspense, based on secret transcripts and unsealed files, Church of Spies throws open the Vatican’s doors to reveal some of the most astonishing events in the history of the papacy. The result is an unprecedented book that will change perceptions of how the world’s greatest moral institution met the greatest moral crisis in history.