Two years ago, I was ready to abandon a biography I’d spent years trying to write when a fellow historian threw me a lifeline. 

The book was about the triple agent Hermann Keller (1905–1970), a Benedictine mole embedded by conspirators against Hitler into the upper echelons of the SS. Keller reported not only to the German resistance but also to the Vatican and the British MI6. In the history of espionage, few spies penetrated deeper into enemy ranks. 

The reason I was on the verge of giving up was that all of Keller’s personal papers had been destroyed during the war—or so I thought. Without them I couldn’t bring him to life, no matter how much evidence I had uncovered. Frustrated, I decided to publish an essay about Keller in the American Historical Review and move on. 

Then, last fall, after the article appeared, I received an email from historian Kevin Spicer, author of Hitler’s Priests: Catholic Clergy and National Socialism. While researching his book, he had come across a small collection of about two hundred handwritten letters written by Keller between 1925 and 1940, mostly to his family and a friend named Katharina Littauer, a Jewish physician from Leipzig. He offered to share them with me. 

The letters changed everything. 

Not only did they bring Keller vividly to life, but they also forced me to question my personal brand of secularism. Through the letters, as well as a collection of sermons he gave after the war, I discovered that Keller viewed Jean-Paul Sartre and his existentialism as every bit as dangerous to the future of humanity as nationalism. Sartre’s philosophy—life as a quest for meaning in a meaningless world—which I first encountered in high school, came to define my intellectual identity through university and graduate school. Because of Keller, I stopped seeing myself as a subject carving out meaning in a godless universe. 

1.

My story with Keller began in 2002, when I discovered a suitcase beside a dumpster on Abraham Lincoln Street in Jerusalem. Inside was a trove of haunting photographs from the 1920s and ’30s, meticulously organized by title and theme. The photographer, Benedict Stolz, a German Benedictine monk at the Dormition Abbey on Mount Zion in Jerusalem, titled one of the photographs The Shadow of Hermann Keller. 

Intrigued, I began writing a biography of Stolz. Keller, I discovered in my research into Stolz’s biography, had been a close friend of Stolz during their youth as students at the monastery of Beuron in southern Germany. He was also, according to historians, a war criminal who somehow escaped justice. Declassified OSS and CIA interrogations of German war criminals who had worked with Keller documented his role during World War II in occupied Paris, where he reported to Reinhard Heydrich’s SD, the intelligence and counter-intelligence arm of the SS. Keller’s assignment in Paris was to spy on the city’s Catholic leaders as well as its leftist artists and writers like Sartre. In my draft, I quoted Charles Glass’s description of Keller in Americans in Paris: Life and Death Under Nazi Occupation as a “psychotic Nazi” who tortured and killed members of the Resistance and sent countless Jews, including the phenomenologist Edith Stein, to death camps. 

By early 2011, I had finished the book on Stolz, which was set to be published in Austria. A few weeks before I was due to return the galleys, I shared them with a monk at the Dormition who had asked to review the manuscript before publication. When he saw what I wrote about Keller, he cautioned me against taking historians at their word. I should talk to someone who knew him before passing judgment.

Galleys in hand, I set off for the convent of Mariendonk on the Dutch-German border, where Keller had worked after the war, to speak with Sister Luitgardis Hecker. 

Upon my arrival, Sister Luitgardis set me straight about Keller, providing irrefutable evidence that he had been a resolute anti-Nazi who fought the regime not only as a spy but also as a historian and theologian. Shortly after Hitler came to power, Keller published an essay whose title translates as “Thy Kingdom Come to Us” in the Benediktinische Monatsschrift, a theological journal read by Catholics throughout Europe, using the Holy Roman Empire as an allegory for Nazi Germany. The essay eviscerated both the Nazis and the ideology of Carl Schmitt, known as Hitler’s jurist. “Between the sword of the state and the cross,” he wrote, countering Schmitt’s notion that religion should serve the regime, “lies a state of perpetual warfare.” Even more daringly, Keller condemned as “satanic” any contemporary political movements that elevate certain races or classes over others. 

During my visit, Sister Luitgardis provided me with enough material to rewrite my Stolz book and to launch a new project focused on Keller. To get me started, she gave me two volumes of his sermons, both published after his death. Because I had written books about Gershom Scholem and understood how he used obscure allegories, allusions, and esoteric references to embed his personal story in his volumes on the history of Kabbalah, Sister Luitgardis trusted I would be able to read Keller’s sermons allegorically. 

Keller, she said, loved teaching the sisters how to use and interpret Scripture to illuminate the postwar reality surrounding them, in particular the ideas of secular humanists like Sartre that were sweeping the continent. A scholar who worked with Keller on a project at Mariendonk, Bonifatius Fischer, put it this way: “His Christian responsibility toward the Word made the spoken word his true medium of proclamation and transmission. He needed the living contact with an attentive audience. He wanted to see where the sowing of God’s Word would sprout and bear fruit. He was a herald and not a writer, a prophet and not a sophist.” 

For a decade, on breaks from teaching and writing a third volume on Scholem, I interviewed people who had known Keller and scoured archives, declassified documents, diaries, and memoirs for fragments of information about my elusive subject. 

In 1936, Admiral Wilhelm Canaris enlisted Keller into the Abwehr, German military intelligence; by the following year he had convinced Heydrich to send Keller to Palestine as a joint Abwehr-SD agent. After France fell to the Germans in the summer of 1940, Canaris persuaded Heydrich that Keller was the most valuable man the SD could place in occupied Paris, now under the military control of Canaris’s fellow conspirator General Carl-Heinrich von Stülpnagel. Heydrich was sold. Fluent in French, Italian, and Spanish, Keller was tasked with disrupting anti-German networks linking Paris, the cultural capital of Europe, to Madrid and Rome. Little did the SD chief know, Keller’s actual mission was to strengthen the very networks Heydrich had assigned him to infiltrate. 

During the war, Keller acted as a spiritual director to members of the White Rose opposition group. He also performed feats worthy of a Spielberg film, such as pulling the Jewish philosopher Anna Reinach, a close friend of Edith Stein, from a cattle car bound for a death camp. Anna and Edith had met in a philosophy seminar taught by Edmund Husserl, the founder of phenomenology. In 1942, with Heydrich at his side, Keller lied to Hitler’s face about Allied military manoeuvres. Two years later, in the summer of 1944, he helped thwart Hitler’s order to destroy the soul of Paris—the Notre-Dame, the Louvre, the Eiffel Tower, and the Sorbonne. A few months later, in November 1944, he played a pivotal role in pushing Himmler to dynamite the crematoriums and gas chambers at Auschwitz-Birkenau. 

It was an astounding story, but all the secondary sources in the world couldn’t tell me why Keller risked his life to save not only friends but also countless strangers. Nor did the sermons help much; as far as I could tell, they were elegant homilies but nothing more. 

Which was where the letters came in. In his correspondence with his family during the 1920s, Keller describes travelling and meeting with the fellow Catholic phenomenologists Dietrich von Hildebrand, Alois Mager, and Daniel Feuling. I knew Keller had known Edith Stein and Anna Reinach; what I didn’t understand until I had the letters was that he was in the centre of a circle of thinkers around Husserl. In fact, he met Katharina Littauer through Adelgundis Jaegerschmid, a Benedictine nun and devoted student of Husserl who rescued his papers from the Nazis in 1939. 

Having studied under Husserl scholar Ernst Tugendhat in Berlin in the 1990s, I was familiar with this circle, whose members broke from their teacher over his focus on the transcendental ego, an abstract idea he used to explain the structure of consciousness. Led by Anna’s husband Adolf Reinach, Husserl’s main assistant, they argued that consciousness was inextricably tied to concrete, lived experience—rooted in embodied, intersubjective, and historical realities. Understanding Keller as a phenomenologist of the Reinach school helped me make sense of his whole life. 

For Keller, belief implied action. I already knew about his actions; his letters now revealed his deeply held beliefs, in particular his theology of the cross. In one letter from 1939, for instance, he used Scripture to express his horror at Kristallnacht to Katharina, paraphrasing lines from the apocryphal book of Judith: “Cursed be the nations that rise against My people. But those who dare speak the truth might end up stoned, sawn asunder, tempted, slain with the sword: they’ll wander about in sheepskins and goatskins, destitute, afflicted, and tormented.” Not only was he divulging his feelings about Nazi Germany; he was also remarking on the likely fate of those who opposed the regime. In another letter from the same period, he alluded to his hope that efforts to overthrow the regime would, at great cost, succeed. “There is an ongoing battle to bring love into the world under threat of extermination. Each action we take brings the Kingdom nearer.” 

For Christmas that year, Keller sent Katharina a crucifix. “For us Christians,” he explained, “Christ’s cross is the true symbol. May the triumphant Christ be your comfort and guide in the year ahead.” A month later, through his contacts in the underground, Keller got her passage to America on a ship from Antwerp. 

The final letter he wrote before his assignment to Paris was addressed to Katharina’s husband, the classical composer Frederick Breydert. With Katharina already safe on the steamer, Frederick was preparing to embark on a journey that would take him across France, to the notorious internment camp at Gurs, and finally to safety in Spain and Portugal. From there he would board a steamer to Brazil, then travel north over land, reuniting with Katharina in New York over a year later, on Christmas Day 1941.

Fully aware of the perils Frederick would face, Keller offered an exegesis of a passage from St. Paul’s Second Letter to the Corinthians, where Paul recounts his own suffering for the sake of Christ. Keller warned Frederick that the months ahead would likely bring a “litany of evils”—misery, envy, sorrow, desolation, poverty, disappointment, betrayal, and the “darkness of confusion and the bitter waters of tears.” Yet, he reminded Frederick, such trials were “the lot of those called to God’s Kingdom.” Keller encouraged Frederick to therefore “bear the heavy burden of exile with Christian patience, striving to bear witness for Christ through action. So, stay faithful and steadfast by offering yourself as a sacrifice, and find joy even in the sorrow of this age. For the storms of this life, though fierce, only hasten the arrival of the Kingdom.” 

2.

In Paris, Heydrich put Keller in charge of spying on Paris’s religious and cultural elite, identifying and suppressing subversive or anti-German ideas, and targeting writers, intellectuals, and artists deemed dangerous to Nazi ideology. His official cover was as a historian and archaeologist working at the German Institute, on the Rue de Lille. 

One of his assignments from the SD was to monitor the archbishop of Paris, Emmanuel Célestin Suhard. He used that cover to collaborate with Suhard, the Spanish consul general Bernardo Rolland, nuns at the St. Agonie convent, and priests at the Spanish mission on Rue de la Pompe to smuggle Jews to safety. One operation involved moving two thousand Jews from detention camps in France to Morocco. 

Keller also helped transform the St. Agonie convent into an MI6 espionage hub for the “Jade” network, which included some fourteen hundred priests, monks, nuns, and laypeople across France. The work was perilous; one of the network’s leaders was tortured and hanged at Buchenwald. 

With the help of Abbé Ferdinand Renaud, an influential Paris priest with ties to the Vatican, Keller developed connections with theologians at the Institut Catholique de Paris, where Jacques Maritain had been a professor before fleeing during the fall of France. In 1932, Stein, Mager, and Feuling were invited by Alexandre Koyré, a Jewish philosopher and historian of science, to deliver a series of lectures at a centre run by Dominican monks in Juvisy-sur-Orge south of Paris, which Maritain attended. Their insights stuck with Maritain. In his 1936 book Integral Humanism, Maritain recognizes “Thy Kingdom Come to Us” as a “daring” theological critique of Nazism and the centralized, all-powerful state. 

Instead of spying on the theologians at the Institute, Keller joined them in discussions of how the church could endure fascism and rebuild society on Christian principles. 

Father Renaud was well connected in Paris’s literary and cultural world. He introduced Keller to the city’s avant-garde, including Sartre and his circle of leftist writers at Café de Flore, the filmmaker Jean Cocteau, and Sartre’s publisher Gaston Gallimard. 

In 1942, Gallimard needed approval from the SD before he could publish Being and Nothingness. The detailed reports Keller had to submit to Berlin about his encounters in Paris were all destroyed during the war, but we know he signed off on the book since Being and Nothingness came out the following year. In 1944, Keller likewise authorized the performance of Sartre’s play No Exit, renowned for its iconic line “L’enfer, c’est les autres”—hell is other people. 

Which isn’t to say Keller agreed with Sartre. Sartre first encountered Husserl through Koyré, a close friend, and spent several years adapting Husserl’s idea of the transcendental ego while drafting his book. The centre of Sartre’s philosophy is the pour-soi, the self-for-itself. Unlike Husserl’s ego, a pure abstraction central to the constitution of meaning, Sartre’s pour-soi is the living, conscious self, defined by its capacity to cast off mauvaise foi—“bad faith,” or self-deception. This self-deception allows individuals to evade the anxiety of freedom and the weight of responsibility in a godless universe. By transcending the inauthentic self, Sartre argues, the pour-soi can fully embrace the burden of absolute freedom and create its own meaning in a meaningless world. 

Keller would have seen the flaw in pour-soi immediately. The pour-soi, he understood from his own training as a phenomenologist, cannot exist in isolation because we are inevitably conditioned by the very structures we seek to transcend. Material, historical, and social conditions, along with our instincts and desires, shape and constrain our actions and choices in ways that cannot be entirely overcome by human will alone. Only grace offers the possibility of liberation from this trap, transcending these limitations to bring true freedom. 

But Keller’s work for the conspiracy didn’t give him the time to counter Sartre. That would have to wait until after the war. 

3.

On July 20, 1944, General von Stülpnagel, thinking Claus von Stauffenberg’s assassination attempt on Hitler had succeeded, used lists compiled by Keller to have his soldiers round up the entire the Nazi leadership in Paris, along with every member of the SS, SD, and gestapo. 

Once word came that Hitler survived, the Nazis began arresting the conspirators. Keller fled into the underground, hiding out at the St. Agonie convent. 

After the liberation of France, Keller left Paris. Through 1945 and most of 1946, he worked on behalf of the Vatican, travelling between Italy, Germany, Switzerland, France, and Spain. His mission was to keep communication open between groups working to rebuild Europe from the ruins of war. 

During this period, Keller watched as Sartre’s misinterpretation of Husserl’s phenomenology transformed into his philosophy of radical liberation, with the pour-soi no longer just the existential individual chain-smoking in a café but reimagined as the proletariat striving to overthrow institutions rooted in bad faith, such as the bourgeois state and the church. Sartre’s political alignment with the Stalinist USSR and the French Communist Party only reinforced this revolutionary ethos. For Sartre, the engaged writer’s words were “loaded pistols,” weapons in the struggle for justice and liberation. 

At the end of 1946, the Vatican asked the bishop of Aachen, Johannes Joseph van der Velden, to find Keller a place to continue his work. Van der Velden installed Keller as the spiritual director of Mariendonk, a convent of academic nuns in a region that had been heavily firebombed by Allied planes and whose ruined cities like Dusseldorf and Cologne were filled with homeless refugees, many of them among the millions of people expelled on Stalin’s orders in the Soviet-dominated East. After more than a decade on the move, Keller finally had a home. 

In February 1947, on the first Sunday of Lent, he donned his black habit and mounted the pulpit in the unheated red-brick abbey church to give the first of several sermons on the mystery of the cross. It didn’t take long for the nuns in the pews, who had already taken workshops with Keller on Edith Stein’s phenomenology, to figure out what he was up to. 

Keller began the sermon by repeating the standard accusations against Christianity coming out of Sartre’s journal, Les Temps Modernes, and the official newspaper of the French Communist Party, L’Humanité. “How can anyone say redemption has taken place at all,” he began, “considering the unfathomable sum of suffering and misery, of guilt and shame, which we experience? Rather, isn’t it the case that over the past two-thousand years since Christ humanity hasn’t gotten any better? That only the methods of criminality have become more refined, their consequences more terrible, human misery deeper? Is all of Christianity not a bitter deception given what we experience daily with our own eyes in which the cruelty of fate stands in no proportion to most people’s actual guilt? What can the thousands of hungry and freezing children begging for bread have done to deserve such a harsh fate? Can we, in the face of their misery, still preach in good conscience that our Lord Jesus Christ, the friend of children, still lives? And if He lives, is it not mockery to speak of His goodness and mercy?” 

These are brutal questions, said Keller, which “shake the very foundations of our faith. But turning away in horror from such questions,” he added, “only proves that one’s religious life and faith stand on the brink of ruin.” 

Keller’s final question of that first sermon—“How should humans live with meaning and purpose in the face of suffering, death, and apparent absurdity?”—would have sounded to his audience like a riff on Sartre’s famous line “Life begins on the other side of despair.” 

And for good reason. In his next sermon, Keller located the source of human misery in what Sartre elevated as the hope for authentic freedom. Sin, said Keller, was precisely how the existentialist defined human action: as the “human longing for self.” In other words, the pour-soi. In our human drama, Keller cast Sartre in the role of Adam, biting from the fruit of the tree of the knowledge of good and evil. 

Far from achieving liberation, through the “rule of the self,” we fall into impossible dilemmas, spawning new forms of alienation and injustice. Efforts at creating meaning are therefore doomed because the divided self, mired in inner conflict, lacks the power to overcome its fragmentation. The human ego, trapped in its contradictions, “descends into a realm that we no longer call ‘world,’ but which we simply call ‘nothingness’ in its lack of essence.” Where Sartre sees being emerging from nothingness, Keller finds only meaninglessness, self-enslavement, and finally self-destruction. The very search for “meaning, purpose and happiness leads to the darkening of the mind.” 

Keller next took aim at the “talk of humanity’s ‘self-redemption,’” a pointed critique of Sartre’s shift from individual existentialism to social revolution. “Humanity, by itself, cannot perform acts of voluntary self-surrender, for it lacks both the motivation and the power entirely.” Such redemption is “utterly futile” because there is no intact “superhuman” capable of carrying it out. Ensnared by their own selfishness, leaders of the supposed avant-garde would inevitably lead societies into new forms of conflict and war. For Keller, Stalin’s gulags and labour camps, the dictatorship he imposed over half of Europe, the show trials, the mass graves and mass expulsions were stark proof that the so-called great liberator was nothing more than a violent fraud. 

Yet Sartre fell for him anyway, because he failed to grasp the theology of the cross. On their own, Keller explained, humans are incapable of overcoming the instinct for self-preservation. Nor are people driven to heroics by abstract ideals divorced from lived experience. Truth could only surpass life itself because truth was alive. It was Christ, Keller said, the “Ego of humanity,” who enabled the acts of self-sacrifice he witnessed during the war. Only Christ is the true pour-soi—in his case, the self-for-others—and only he can counter human selfishness. As Keller often quoted from Paul’s Letter to the Galatians, “It is not I who act, but Christ in me.” 

Keller illustrated this with the image of Christ before the Nazi People’s Court: 

This, Keller believed, was how the followers of Christ stared down the Nazis. He may have been thinking of the young resisters Sophie and Hans Scholl bravely facing the guillotine, or of the dozens he knew who went to the gallows, when he wrote, “The cross alone is our protection against the demons who seek to intimidate us with the threat of violence, filling us with fear and terror. . . . Though darkness still envelops entire regions of our inner and outer being, and the demons still rule over whole provinces, we best banish the demons with the cry ‘Hosanna,’ ‘Kyrie eleison.’ Calmly and fearlessly, and with full awareness of our weakness, we go forth to meet the coming King. We know He will arrive, and this hope does not abandon us. It is not joy in the destruction of this world that drives us to call out to Him, but the joy that shines brighter within us as we praise His greatness: ‘Hallowed be Your name!’” 

4.

One of Keller’s most ardent admirers was Bishop van der Velden. Having lost an eye during the war from a gestapo beating, the bishop understood the risks Keller had taken in the underground. By the 1950s, he began urging Keller to publish his account of the war. Out of France, particularly, fabricated reports in the Communist press accused Keller of crimes against partisans. 

Keller demurred, not least because defending himself would mean people might view him as a hero, which would amount to a denial of Christ working through him. 

In a letter to Anna Reinach shortly before her death in 1953, Keller repeated this theological explanation for his silence. Anna had written to thank him for spiriting her out of Germany. True to form, Keller waved off claims of heroism. “My life’s greatest blessing,” he wrote, “has been that the Lord God has trusted me to carry the load that comes from His own hand.” 

While working on this book, I realized I couldn’t do justice to Keller’s theology without undertaking a close reading of the New Testament. When I returned to his sermons after reading Paul’s letters, it struck me that they read like epistles. For Keller, as for Paul, theology was inseparable from the world around him. In his sermons, he often referenced contemporary politics and other writers, such as Karl Barth and the expressionist poet Ernst Toller. But Keller wasn’t trying to impress those in the pews with his erudition. His goal was to strengthen their faith. And it worked—at least on me. Twenty years after pulling those glass plates from a garbage heap in Jerusalem, I now say, “It is no longer I who live, but Christ who lives in me.”