Written in Latin for students at the Gregorian University in Rome, Bernard Lonergan's De Deo Trino (The Triune God) is a monumental two-part examination of trinitarian theology published initially in 1961 and again, in revised form, in 1964. The first part, the pars dogmatica, is here translated into English in an edition that includes the original Latin on facing pages. The work begins with the Prolegomena, which traces the dialectical development of trinitarian doctrine by Christian thinkers from the time of the New Testament to the Council of Nicea (AD 325). Following is a discussion of five theses outlining the evolution of the principal features of trinitarian doctrine from the New Testament through the patristic era. Along with its companion volume on systematics, The Triune God: Doctrines represents the most comprehensive treatment of trinitarian theology in recent centuries. This English translation ensures that Lonergan's masterpiece will at last be available in its entirety to contemporary readers.