A French Dominican priest who become one of the most influential theologians of the 20th century on the topic of the Roman Catholic Church and ecumenism.
Congar offers an insightful and neatly articulated introduction (on the spirit of ressourcement) and conclusion (towards the future of ecumenism). Unfortunately, apart from a beautiful explanation of the value of "the Church Fathers" as distinct from early Christian writers, I found most of the middle less than compellingly developed, at times opaque, and somewhat forgettable.
While this is a light text from him, on a first read, I would not consider this essential Congar. One of his most central and driving themes here—the vital, even enormously helpful distinction between Tradition and traditions—is one he develops elsewhere with greater focus and perhaps clarity.