This book is the culmination of a long companionship, a final link between a historian familiar with theology and a theologian keen on history. It was in February 1966 that Etienne Fouilloux met the Dominican theologian Yves Congar for the first time. He then began a thesis on the origins of ecumenism. Congar liberally opened his personal archives to him. For fifteen years, Congar did not leave the horizon of Fouilloux. Congar attended the defense of his thesis in 1980. Then, according to the work of the historian, the theologian was never far away, voluntary or involuntary protagonist of many of his studies on the theological crises of the 1930s and 1950s, the Second World War or the Second Vatican Council.In scattered but recurring touches, Fouilloux had already shed light on many aspects of Congar's work, including by publishing Journal d'un theologien. 1946-1956 (Editions du Cerf, 2000). Today, an overall plan and the cement necessary for writing a life story conceal the many stones previously brought to the building and finally constitute a biography of Father Congar. The sum is undeniably greater than the addition of the parts.Sabine Rousseau, Archives de Sciences sociales des religions, October-December 2021