Over the past two years, Cardinal Walter Kasper has earned the un­official title of chief theologian of Pope Francis. At Francis’ invitation, he addressed the February 2014 consistory of the College of Cardinals on a theology of the family, and the Pope has praised the book on mercy that Kasper gave him a few days before his election. Many will welcome the publication of this long-awaited work of ecclesiology. For two decades, Kasper had been forced to put off the completion of this final volume in a trilogy in systematic theology begun with Jesus the Christ and The God of Jesus Christ. The concept of “communion”, which Kasper claims was the “central concern and main motif” of the Vatican Council’s ecclesiology, provides the thematic core of this vo