“In putting the doctrine of the Trinity at the head of all dogmatics we are adopting a very isolated position from the standpoint of dogmatic history.” (I/1, 300) Barth noted that usually the place belongs to the doctrine of the Holy Scripture “as the principium cognoscendi (apart from the actual content of faith)”, that is, the method which is supposed to be independent of the content. Or, even if the doctrine of God is discussed, Barth added that usually one would “deal first with God’s existence, and nature and attributes.” Then I stumbled upon the table of contents of Michael Horton’s new book, The Christian Faith, and somehow it proved Barth’s comment, as the discussion on the Trinity followed the discussion on the (communicable and incommunicable) attributes of God.
Barth, on the other hand, put the doctrine of the Trinity at the head because that’s the way biblical witness to revelation of God worked. The biblical concept of revelation is trinitarian, as in revelation there are three simultaneous elements of unveiling (as something is really and truly unveiled), veiling (as unveiling does not mean complete unveiling), and impartation (that this unveiling must be imparted concretely to human in order to become true unveiling), and that these three elements of revelation corresponds with the Son, the Father, and the Spirit. That’s why Barth concluded that the root or ground of the doctrine of the Trinity is the biblical concept of revelation, although we have not delved into the doctrine itself.
Barth started with the Son, and it corresponded with the way that the doctrine of the Trinity, biblically and historically (as in the controversy leading to the Council of Nicaea), arose from the question of the identity of Jesus. The doctrine of the Trinity was developed from the confession that “Jesus is Christ” or “Jesus is Lord.”
In summary: God reveals himself as the Lord, in the name of the Father, the Son, and the Holy Spirit.