Deum nemo vidit unquam… Ipse enarravit. “The heart of every culture lies in its approach to the greatest of mysteries: the mystery of God” (John Paul II, address to the UN, Oct. 5, 1995). What is the Christian approach to the mystery of God? We can say that it is an approach that starts from a solid base, namely, the Revelation that God has made of himself in Jesus Christ, in his Church. But this mystery, precisely because of its complexity calls for a diversity of approaches. In the treatise on the Triune God, we will begin with the actual faith about the mystery of God: what do we mean when we say “I believe in unum Deum”? But in order to give a satisfactory answer we will start the treatise from the mystery of God in the human heart and, in a second step, move on to review the other influences Christianity has received in the elaboration of Trinitarian theology: universal human experience, the presence of God in Sacred Scripture and the various formulations of Trinitarian dogma throughout history. The central part of this treatise will, in each case, be systematic reflection on the mystery of God revealed in Jesus Christ.