Why have theologians in the classic tradition of trinitarian doctrine found it easy to confess that the external actions of the Trinity are undivided? One reason is that they started from a clear confession that the internal actions of the Trinity were not undivided. Or, to put it less double-negatively, the internal actions of the Trinity are distinct and distinguishable as real relations that stand in relative opposition to each other.
Let me take that a little slower. If we’re going to use the concept of “action” to talk about what God does, we’re going to say that God is the source of all sorts of things in the world. But then if we turn around and ask about what God is doing when considered apart from these doings in the world, we have a choice to make. One option is to say that in the divine life there is no action, only being. And then we could try to describe be-ing as something very alive, and as something greater than action, while carefully avoiding the word “action” because we want to save it for what God does with the world. You can go pretty far with this option. Question: What’s God doing when he’s not doing anything? Answer: Be-ing, but in a divine way. Apophatic silence descends a bit prematurely.
Another option is to apply the category of action to the divine life in itself, and try to say what those actions in the life of God are. And this is the path that the main stream of trinitarian theology in fact pursued. Building on what Augustine called opera and the Cappadocians called energeia, Latin-language theology developed a distinction between the inward acts of the Trinity and the outward acts. What are the inward acts of the Trinity? They are generation and procession, concepts which had long been fundamental to trinitarianism, but which now came under the general conceptual framework of “actions” in God.
Notice the key thing about these actions: each of them has a person of the Trinity at each end. The Father begets or generates the Son, which puts Father and Son at opposite ends of the relation. The Spirit proceeds from the Father (“at least from the Father,” we can ecumenically agree, prescinding for now from filioque), putting Spirit and Father over against each other within the divine life. This polarity or opposing-relation is why the inward works are not called undivided: they mark the distinctions among the persons. The formula used by the Council of Florence in 1439 is what has become the classic statement of the principle: In Deo omnia sunt unum, ubi non obviat relationis oppositio: “In God all things are one except where there is opposition of relation.” (Another aside for Orthodox readers: even though this is Florence 1439, the principle of opposition itself can easily be found in Nazianzus and company. What’s helpful at Florence is the clarity of the terminology. This is not a sucker punch for double procession.)
A very helpful discussion of this is in Pannenberg’s Systematic Theology, volume 2. According to him, one of the main advantages of distinguishing between internal and external actions is to recognize something about divine aseity: the notion of internal actions is “a great gain for the actual understanding of God that God should be thought of as active.” He sets it up this way:
Does there not have to be a world of creatures, or a relation to it, if God is to be thought of as active? Christian doctrine denies this by describing the trinitarian relations between Father, Son, and Spirit as themselves actions. To these divine actions in the creation of the world are added as actions of a different kind, as outward actions. (ST 2:1)
And Pannenberg calls for a high wall between internal and external:
The acts of the trinitarian persons in their mutual relations must be sharply differentiated from their common outward actions. This differentiation finds support in the rule that posits and antithesis between the inseparable unity of the trinitarian persons in their outward action relative to the world and the distinctiveness of their inner activities relative to one another, which is the basis of the personal distinctions of Father, Son, and Spirit. (ST 2:3)
In other words, external acts of the Trinity are undivided because the internal acts of the Trinity are distinct relative to one another. Because this is true, we can recognize that “God does not need the world in order to be active. He is in himself the living God in the mutual relations of Father, Son, and Spirit. He is, of course, active in a new way in the creation of the world.” (ST 2:5)
So the internal actions of the Trinity help us conceive of God in himself as the living and active God, not a God waiting for a created, historical stage on which to be living and active. They enable a confession of dynamism as part of the divine life, as a form that aseity takes. And they do this quite tidily, without illegitimately manufacturing any new content for trinitarian theology. Why? Because the content continues to be what it has always been: the generation of the Son and the procession of the Spirit. Anchoring the livingness and activity of God in eternal generation and eternal spiration, the older theology had the leisure to declare the external works of the Trinity undivided. The pressure was off: Father was Father because he Fathered the Son, and so on.
By contrast, anybody who denies or downplays the eternal generation of the Son is likely to need the historical manifestation of the incarnate Son to carry all the meaning and significance. Show me a theologian with a weak grasp of the internal actions of the Trinity, and I’ll show you a theologian who makes too much of the separateness of the external actions. Such a theologian is bound to milk the external actions, and is under considerable pressure to read them as three different agents doing three distinct things. In extreme cases, for example Moltmann at his most cruci-drastic, the events in the history of salvation may turn out to be the actual ground of the distinctions among the persons of the Trinity. I understand the desire to say the cross is where the action is. But if you fail to recognize that the action was in the being of God before it was among us, you give away too much. As Karl Barth asked Moltmann in a 1964 letter, “Would it not be wise to accept the doctrine of the immanent trinity of God?”