With a snappy title like, “God’s Eternal Self-Consciousness,” you know this post is going to be good! An alternative title could have been, “The Aseity of a Personal God,” which is just as catchy!
If you have ever struggled to explain how the Trinity is necessary for a personal God, then this is what you say:
The idea of the Trinity of essence is one with the idea of the Divine personality; and, therefore, to have an ontological conception of the essential Trinity is to have a conception of the form which is fundamental and necessary to the personal life of God; is to have a conception of those momenta of the essence of God, without which personality and self-consciousness are inconceivable. It is true, both ancient and modern Arianism is of opinion that God may be a personal God without being a Trinity, and that the personality of God is sufficiently secured if we represent to ourselves a “God the Father,” to whom we attribute self-consciousness and will . But we ask, — is it possible for us not merely to imagine to ourselves, but to think, that God could have been from eternity conscious of Himself as a Father, if He had not from eternity distinguished Himself from Himself as the Son, and if He had not been as eternally one with the Son in the unity of the Spirit? Or, in other words, Is it possible to conceive of God as eternal self-consciousness without conceiving of Him as eternally making Himself his own object? When, therefore, following in the footsteps of the Church, we teach that not merely the Father, but also the Son and the Holy Spirit eternally pre-existed and are independent of creation, we say that God could not be the self-revealed, self-loving God, unless He had eternally distinguished Himself into I and Thou (into Father and Son), and unless He had eternally comprehended Himself as the Spirit of Love, who proceeds forth from that relation of antithesis in the Divine essence.
[Hans Martensen, Christian Dogmatics, p. 107]
This is applicable to how God can be personal while preserving his aseity and freedom vis-à-vis creation. In a Unitarian reckoning, you can have a personal “Father” God in relation to the creation, but then creation would be necessary for this personal dimension of God. By contrast, if God is self-constituted in his essence as an I and Thou, then he can exist in free relation to his creation as the One who has life and fellowship in himself (using Barth’s language).
However, Martensen turns to the love of God as the basis for establishing a certain necessity between God and the world for God’s perfection (see below). So, whereas Martensen does not use the personal relations of God per se to make such a connection, he turns to the attribute of love, established by these personal relations, in order to do so.
Martensen has an intriguing account of the Holy Spirit as the free, ethical relation of God to himself, which would not be the case if his love terminated in the Son alone. And it is this free “procession” within the Triune God that is the basis for creation ad extra (see p. 110). It is at this point where Martensen ventures into questionable statements about the necessity of creation for a loving God, which he has avoided heretofore. For example, he states that “perfect love is not merely the love of God to Himself, to His own perfection, but must also be conceived as love to what is imperfect; in other words, it must be conceived as the will to create a world, one of whose essential features is the need of God…” (111). The divine blessedness becomes perfect only when the grace and love of God are fulfilled in the “glorious liberty” of his children:
[The divine blessedness] then for the first time becomes perfect, in so far as it is the will of God not merely to rest in His eternal majesty — for in this the Triune God was able to rest independently of the world, before the foundations of the world were laid; but to rest and be blessed in the completed work of grace and love, in the glorious liberty of the children of God, — a goal which will not be reached until, in the words of the Apostle Paul, God shall be all in all. [112-113]
So, God is able to rest eternally in his “majesty,” but that would be less than the “blessedness” of revealing and consummating his love to a dependent creation. As such, an attribute (or perfection) of God requires the existence of a creation ad extra. For some of us, usually Reformed sorts, that would undermine the freedom of grace, toward creation, by turning it into a necessity for God’s perfection. Not surprisingly, he faults Calvin for not recognizing this mutually constituting relation between God and his free creatures (p. 115).
However, Martensen tries to avoid the Reformed criticism, as I see it, by designating this “lack in God” as a “superfluity”:
In a certain sense one may say that God created the world in order to satisfy a want in Himself; but the idea of God’s love requires us to understand this want as quite as truly a superfluity. For this lack in God is not, as in the God of pantheism, a blind hunger and thirst after existence, but is identical with the inexhaustible riches of that liberty which cannot but will to reveal itself. From this point of view, it will be clear, in what sense we reject the proposition, and in what sense we accept it, “without the world God is not God.” 
Whether this notion of a “superfluous” necessity is convincing, the reader will have to decide. I have my doubts. Nonetheless, the doctrine of the Trinity in Martensen’s dogmatics is highly stimulating and beautifully expressed. I recommend it to one and all!
Image: Trinity Window at St. Mark’s Lutheran Church, Asheville, North Carolina