In previous entries in what is becoming an impromptu antiphonal blog series on the Trinity, Fred Sanders and I have focused on the nature and relevance of the doctrine of inseparable operations (see here, here, and here). To this point, we have considered ways in which the unity of God's being informs the unity of God's action towards his creatures in making, redeeming, and perfecting them for his glory. In the present post, I want to consider a couple of the ways in which his tripersonal manner of existing is inflected in his external works. As God is one and three, so God acts as one and three.
God's triune identity informs our understanding of God's triune actions in two areas, both which specify in different ways how the three persons relate to one another within the context of their indivisible activity toward creatures.
(1) The doctrine of appropriations helps us appreciate why the Scriptures characteristically appropriate (for example) the act of predestination to the Father (Eph 1.4-6; 1 Pet 1.1-2) even though each divine person is an agent of God's electing grace (John 6.70; 13.18; 1 Cor 2.7-11). The reason distinct divine actions are appropriated to distinct divine persons is not because God's actions toward his creatures are divided between the persons: the external works of the Trinity are undivided (opera Trinitatis ad extra indivisa sunt). The reason is due to the ways in which the personal characteristics of the three manifest themselves in their common, indivisible action. Thus, as the Father is the principle of the Son and the Spirit (i.e., he eternally generates the Son and he eternally breathes forth the Spirit), his personal character shines forth in a special way in predestination, the principle act of the Trinity in salvation. Similarly, because the Son is eternally generated by the Father and because he eternally breathes forth the Spirit, the Son's personal character shines forth in a special way in the work of redemption, since the work of redemption flows from divine predestination and issues in the work of sanctification (Eph 1.3-14). Finally, because the Spirit eternally proceeds from the Father and the Son as the bond of God's tripersonal perfection, his personal character shines forth in a special way in the work of sanctification, since the work of sanctification brings the acts of predestination and redemption to their divinely appointed goal (Eph 1.4; 5.27), making us a habitation for the triune God (John 14.16-17, 23).
More clearly than in the doctrine of appropriations, (2) the doctrine of divine missions reveals how the mystery of God's tripersonal being shines forth in God's tripersonal actions toward his creatures. In trinitarian theology, "mission" refers to the "sending" of one divine person by another for a specific purpose in relation to our salvation. In Scripture, these missions follow a very specific pattern: the Father sends the Son to accomplish his redemptive mission; and the Father, with the Son, sends the Spirit to accomplish his sanctifying mission (Gal 4.4-7; John 15.26). This missional pattern in turn corresponds to the eternal relations that constitute the divine persons: the Father eternally generates the Son; and the Father, with the Son, eternally breathes forth the Spirit. As insightful as this correspondence is, it does not fully capture the wonderful reality expressed in the doctrine of the divine missions. Not only do the eternal relations of the Trinity constitute the "whence" of the divine missions, the latter being the temporal embassy and extension of the former. The eternal relations of the Trinity also constitute the "whither" of the divine missions insofar as they provide the divine prototypes and goals of those missions: the goal of the Son's redemptive mission is to make us sons and daughters in order that he might become the firstborn among many brothers and sisters (Gal 4.5; Rom 8.29); the goal of the Spirit's sanctifying mission is to embrace us within the fellowship of the Father and the Son, pouring out the Father's love into our hearts (Rom 5.5), and awakening within us the Son's filial cry of "Abba! Father!" (Gal 4.6). Formally stated, we may summarize the law of God's triune action as follows: The eternal relations of the Trinity are inflected in their undivided external operations, even as the external operations of the Trinity extend their eternal relations to elect creatures in a manner suitable to creatures.
Why does any of this matter? Along with the fact that the doctrines of inseparable operations, appropriations, and missions help us think truly about the true and triune God in his saving action, these doctrines also help us become better readers of Scripture (see, for example, here) and help deepen our communion with the triune God (see, for example, here and here). Those are not insignificant reasons for caring about trinitarian theology.
Now, in order to round out this impromptu series, somebody needs to say something about the relationship between the Trinity and divine simplicity and about the nature and significance of the eternal generation of the Son of God. Fred?