For Barth, the essential and most important question in theology is: who is God? Yet for Barth, this question cannot be answered apart from two other questions, namely: what does God do? and what is the effect? It is in these three questions and their answers that Barth finds the doctrine of the trinity. Namely, God is the Revealer, Revelation itself, and the Revealing.
Now, Barth is careful to say that the trinity is not strictly biblical, by which he means we find no explicit doctrine of the trinity in the Bible. He does believe that the Bible prefigures the trinity, but does not develop this. But he does say that the biblical attestation to revelation is the "root" of the trinity. That is, for Barth, what the Bible says about revelation necessitates the doctrine of the trinity. Therefore, the doctrine of the trinity is church work, it is exegesis, it is analysis of what one finds in the biblical attestation to revelation.
Then Barth goes on to develop his doctrine of revelation. He uses the following definition: "Revelation in the Bible means the self-unveiling, imparted to men, of the God who by nature cannot be unveiled to men." (CD I.1 §8.2 p. 315). Barth then elaborates on this definition in three parts:
1) In the first part, Barth concentrates on the self-unveiling of God. He does this by saying that God differentiates himself from himself, he takes a step toward the event of revelation. As Barth writes:
"He himself must make a step towards this event; to the extent that this step obviously means something new in God, a self-distinction of God from himself, a being of God in a mode of being that is different from though not subordinate to his first and hidden mode of being as God, in a mode of being, of course, in which he can also exist for us." (CD I.1 §8.2 p. 316).In this self-unveiling, God steps out of eternity into temporality in the incarnation of Jesus Christ. Jesus Christ is Revelation, is the self-unveiling of God.
2) In the second part Barth focuses on the last section of the definition, namely "the God who by nature cannot be unveiled to men." Here Barth is emphasizing the hiddenness of God. Even by taking temporal form, this form does not become the possession of humanity. Even in concrete form, God is free to reveal or conceal as he wills. Even in this form, God always remains a mystery. As Barth writes:
In it [temporal form] God cannot be grasped by man or confiscated or put to work... if this were so, the revelation in question would not be that of the God who by nature cannot be unveiled to man. We should simply have one of those mysteries that one day unveil themselves to us and are mysteries no more. The mysteries of the world are of such a kind that some day they can cease to be mysteries. God is always a mystery." (CD I.1 §8.2 p. 321).3) In part 3, Barth focuses on the middle term in the definition, "imparted to men." This term, for Barth, means that God's revelation is an historical event. Yet, he does not label this "historical" as other historical events are "historical." That is, this event cannot be grasped by humans. It comes at specific times to specific people. That is why Barth labels it as historical. That revelation is historical means that it came to specific people at a specific time and place, and it cannot be repeated.
My mind is buzzing at this point and is too full, so I leave with one comment.
Barth moves very quickly to the doctrine of the trinity, almost as a presupposition of Christian dogmatics, without much argumentation or support, as if, given the text of the Bible, the doctrine of the trinity is a necessary and immediate consequence of reading the Bible. Yet, Church history clearly puts that assertion to lie in the fact that Christians used many different formulations of the relationship of God, Jesus, and the Holy Spirit in the first four centuries of the church before formalizing the doctrine of the trinity in the fifth century. Perhaps Barth will come back and argue and explain the trinity more fully later.