This is a New Translation of a Book in which the author affirms that the Trinity -- one God in three persons, Father, Son, and Holy Spirit -- is the central mystery of the Christian faith that enlightens all other mysteries of the faith. An understanding of the Trinity is essential for the protection and defence of the divine/human person of Jesus Christ and his saving work. This book documents the historical development of the doctrine of the Trinity and its significance for the Christian faith. In a broad and systematic way, Ladaria traces the debates within the early Christian church as the concept of the Trinity developed. He also reflects on the great masters of church tradition concerning their understanding of the trinity and is in dialogue with various contemporary theologians on this subject. This book has been written especially for theological students and professors as an aid in grasping the Trinitarian formulation both historically and theologically and to assist in deepening their faith and reflection in the mystery of God.