This is Part IV in my series analyzing Luther's Catechetical Doctrine of the Trinity. It's really a continuation of Part III.
Part I. Part II. Part III.
We ended last time promising more on Luther's break with medieval trinitarian speculative theology.
Over time, after the brilliant work of Augustine, the Trinity was marginalized in Western Theological thought. The Trinity lost its constitutive role in the presentation of the Christian faith. The existence and attributes of God de deo uno edged out discussions of God de deo trino.
Post-Augustinian Trinitarian theology tended to be more interested in developing a rational justification for belief in the Trinity independently of the revealed work of the Godhead in the economy of salvation. The threeness of God becomes a complicating factor, a puzzle that must be reasoned through. [BTW, I'm not taking sides in the debate about whether Augustine's doctrine of the Trinity caused this state of affairs in medieval theology. I have some concerns with, for example, Gunton's account of the genesis of the problem. My only point here is that there was indeed a problem with the church's theological formulation of the doctrine of the Trinity.]
Luther brilliantly recovers the constitutive place of the Trinity for the understanding the Christian faith. If Augustine’s ontological speculation set in motion a trend in Western theology to elevate the divine substance as the presupposition of divinity, the highest ontological principle, then, Luther emphatically breaks with that tradition.
The Trinity is for simple Christians who trust in the Persons of the Father, Son, and Holy Spirit, not primarily for philosophers and scholastics who are interested in logically penetrating problems arising from meditation on the substance of God. The Trinity is the gateway through which one encounters God in the Person of Father, Son, and Holy Spirit.
Otherwise stated, the Trinity simply is not a problem for Luther. God the Father, Son, and Holy Spirit have no need of metaphysician-theologians to justify their existence philosophically. They have revealed themselves in their works. Luther has no interest in what had become a preoccupation in late Medieval Trinitarian theology in the West—logical method, systematic cohesion, terminological precision, and so on. Medieval theologians tended to downplay the existential and soteriological import of the doctrine of the Trinity. They were concerned with unpacking the logical and metaphysical implications of God’s threeness and oneness. These philosophical interests left the nature of the Triune God’s relationship pro homine underdeveloped (at best). The Patristic origin of trinitarian theology as a soteriological development was lost. In the Creed, however, according to Luther, we learn “to know God perfectly,” which means that we learn “what we must expect and receive from him.”