Muller, Richard. The Triunity of God. Post Reformation Reformed Dogmatics, volume 4. Grand Rapids: MI, Baker Academic.
Given that there aren’t many specifically Reformed constructions of Trinitarianism, I would say that this book fills a woeful lacuna. However, since it has long remained out of print, it doesn’t (and don’t tell me the age-old narrative that Baker “soon plans to republish it”). Nevertheless, as JI Packer said of Herman Witsius, this book is mind-forming. See the notes here.
Muller begins in the Middle Ages with Boethius’s classic definitions. The problem with Boethuis’s definition of person: The definition ultimately poses all manner of problems for the doctrines of Trinity and Christ when the concept of individual substance is taken to indicate a unique entity essentially distinct from other similar entities” (Muller 27).
Latin authors preferred to speak of the Father as principium rather than cause, unlike the Greeks. An efficient cause, for example, is perceived of as a different substance than its effects (Muller 47)! Aquinas’s denial of real distinction is a denial of a substantial distinction. He wants to deny that any distinction that would make the essence one “thing” and the “persons” other “things.”
Structure of the Book
Clarifying medieval discussions on filioque: all Westerns agreed that the Spirit proceeded from Father and Son as from one principia. Causal language was eventually abandoned, for it implied the Son/Spirit to be of a different substance (effects are not the same substance as causes). Further, and right before the Reformation, the Trinitarian life ad intra was lining up with the work ad extra (Muller 59).
The Reformation forced thinkers to restate the doctrine of the Trinity anew. Advances in historical criticism and typology meant that some exegesis needed revisiting. Muller notes three basic issues: the inheritance of Patristic vocabulary, renewed exegetical battles against the Socinians, and a new philosophical vocabulary (62).
Objection: does essential identity demand personal identiy? The Reformed generally respond that this is true for finite essences (Muller 211). The orthodox are slowly moving away from the old Cappadocian argument of three men having the essence of manness. The problem is that this moves from “genus (man” to “Genus (God)”, yet God isn’t a genus.
Nor is it a quaternity: the three persons plus the one essence. Persons and essence are not distinct as a thing (res).
Exegetical Issues and Trajectories
The Reformers assumed a hermeneutic of movement from shadow and promise to fulfillment (214).
Eternal decree and election of Christ. God works either by his decree or the execution of it (Perkins). As the Reformed saw that this was Trinitarian, they began to see the covenant of redemption.
The order of the persons ad intra in the opera personalia is mirrored ad extra in the opera appropriata (Muller 268). These are modes of operation contributing to the ultimately undivided work of the Godhead ad extra. The works of the Son and Spirit terminate on their persons. By terminate we mean the terminus is paired with a fundamentum. This pair means a relation of acts bringing about relations (268). The fundamentum is the source; the terminus is the conclusion of the action constituting the relation.
Aseity of the Son
The issue: Calvin denies explicitly that the Son is from the Father “with respect to his eternal essence” (Muller 325). The Son is generated per Sonship, not divinity.
However, Ursinus: the essence is absolute and communicable. The person is relative and incommunicable.
Arminius rejected Calvin’s view, insisting that “Christ, as God, has both his sonship and his essence by generation” (329).
This is not to say that every single construction is satisfactory. However, the Reformed orthodox did provide a robust Trinitarian framework that avoids most of the difficulties and charges labeled at scholasticism.