Introduction is paper explores the potential impact of Fr. omas Joseph White’s impressive new book on the Trinity for the ecumenical dialogue between the Roman Catholic and the Orthodox Churches. 1 In doing so, the paper responds to the editors’ kind request for an explicitly ecumenical approach to the book. erefore, this paper concentrates on the issue of the Trinity from an ecumenical perspective. But this is not a simple task. Before reading the book, the reader might have thought that Eastern and Western theologians had somehow reached a gentlemen’s agreement that the insertion of the Filioque clause in the Nicene-Constantinopolitan Creed had happened sometime somewhere in the Wild West (namely, at the local Council of Toledo in 589). Building on an honest misunderstanding, the Latin Church gradually added to the initial statement of the Nicene-Constantinopolitan Creed, according to which the …