“The external works of the Trinity are undivided,” says a classic principle of trinitarian theology. Whatever God does outside of the divine essence, the Father, Son, and Holy Spirit do as the one divine causal agent.
This is great news for monotheism, because if three distinct divine agents were bringing about created effects by distinguishable actions of efficient causality, that would be hard to tell from a polytheistic account of divine action. If three divine persons were doing distinct things, it would be possible for there to be one God behind it. We might be able to take recourse to some revealed assurance that the three doers were actually one God “behind the scenes,” but on the scene they’d be three.
On that account (which I am about to disavow), the Christian story would still be wonderfully preferable to a pure polytheism: Zeus, Hera, and Poseidon do distinct things that compete with each other, sometimes with spectacular results (“I’m for Troy, you’re for the Achaians;” “I’m for Odysseus but you’re against him”). By contrast, there is a strongly concerted and cooperative force to the actions of the Father, Son, and Holy Spirit: more like “You be the dove, I’ll be the voice, and together we’ll baptize Jesus in the Jordan.”
We could still say that the whole Trinity did that baptism, but (on this account, which I may have already mentioned I’m about to disavow) we could also pick out specific configurations of matter and energy which we would identify as the distinct work of a particular person of the Trinity, exclusively: Only the Father caused the sound waves that were the voice that resounded in the sky and was heard at the Jordan; the Spirit as Spirit did not pronounce those words, blow the sonic waves that wiggled those earbones, or otherwise affect the timbre of the audible voice. Ditto for the dove descending (feathery but not fatherly), same for the man in the water (Son solo).
Surely, the argument (the one I’m about to disavow) goes, what we have in the baptism of Jesus is the voice of the Father alone, the dove of the Spirit alone, and the incarnation of the Son alone. Surely that’s the common sense way to read the story, and only the most inelastic neoscholastic could try to strain this lovely, lively, three-personed story through a screen marked “opera trinitatis ad extra sunt indivisa” (external works of the Trinity are undivided). To explain this dramatic epiphany of the three persons as one tripersonal God working a dove while three persons undividedly cause the voice must surely be tantamount to explaining it by explaining it away. Surely that’s insufficiently trinitarian. Surely that’s an unbaptized God-concept right here at the point where God in person is being baptized in the Jordan.
But surely not. (Discerning readers will note the promised disavowal beginning here.)
That the external works of the Trinity are undivided means that Father, Son, and Holy Spirit concur in every work. They don’t concur merely by division of labor, claiming particular parts of a collaborative project; they concur more deeply. They concur in every part of every work. Every single created effect worked by God is worked by the three as one.
So let’s say that was a physical, biological dove that descended at the Jordan. The Gospel accounts are not precisely detailed about this: it could have been a visionary phenomenon, it could have been a dove-like motion of something else (“Spirit descending like a dove,” Mark 1), etc. But if it was a real, live dove of the Aves Columbiformes clade, then it was a creature. Perhaps it was spoken into existence just for this event before vanishing away, exnihilated and annihilated ad hoc at the Jordan that same day. Perhaps it was a normal, local dove, deputized for this one special performance, who then flew away to live out its natural lifespan somewhere near Petra. On questions like these, the four most important words to utter are “we do not know.” But while the descending dove bears the meaning “Holy Spirit,” it’s a pretty safe bet that it was not an instance of dove nature taken up into hypostatic union with the third person of the Trinity. In other words, the creaturely dove was not the Holy Spirit in that strong sense. It signified the Spirit. But if it was a creature, it was a creature of the creator, and the creator is Father, Son, and Holy Spirit.
The same considerations apply to the voice from heaven. It is a created effect of some kind, though to all sorts of questions about its acoustics we would have to answer “we do not know.” It says “this is my Son,” so it signifies as its speaker the Father alone, not the Son nor the Spirit (neither of whom have Sons about whom they can say “my”). But as a physical phenomenon within creation it is a movement of the air, a vibration that propagates through a material medium, and it is caused by God.
So as Augustine says (On the Trinity book 4), “the Trinity together produced both the Father’s voice and the Son’s flesh and the Holy Spirit’s dove, though each of these single things has reference to a single person.”
But wait, the man in the water: surely that is just the Son, and I mean actually is the Son, not just signifies the Son. Isn’t the incarnate one there, the one on whom the dove descends and about whom the voice speaks, the one who is the hypostatic assumption of human nature into personal union with the second person of the Trinity, and precisely not the first or third persons? Doesn’t the failure to affirm that Jesus is exclusively the Son entail patripassianism and pneumato-incarnationalism and other terrible heresies that don’t even have names?
Yes, the incarnation is the incarnation of the Son only, and not of the Father or the Spirit. Jesus is not the Father, nor is he the Spirit. The Son alone is incarnate. Yes, yes, a thousand times yes.
The incarnation is of the Son, but it is by the Trinity. That is, Father, Son, and Spirit brought about the incarnation of the Son. Sixteenth-century theologian Wolfgang Musculus said, “the three persons of the holy triad effect this work of incarnation, but only one truly puts on the flesh.”
There is a rich discussion of this in patristic and medieval theology, but the best place to pick up a detailed account of it is probably in Richard Muller’s Post-Reformation Reformed Dogmatics, volume 4 (starting around page 255), where thinkers like Musculus, Vermigli, and Polanus have already summarized and synthesized the long tradition of reflection.
Peter Martyr Vermigli recognized that it was necessary to think of the incarnation not as an exception to the indivisa rule, but a strong instance of it. According to Muller, Vermigli
distinguished the incarnation into two categories of divine operation, the divine actio and the resultant divine opus. The actio, or action, of God in the sending of the Son is clearly understood by Scripture as an action of the Father and the Spirit, and, indeed, of the Son himself as well: ‘efficiens enim causo et actio, ad res personas petinuit.‘ Vermigli acknowledges the difficulty of identifying the Son both with cause and effect in incarnation, but he insists that this must be done –the Son is both sender and sent.
Vermigli himself goes on:
Christ alone took to himself our human nature, but the efficient cause of the action is the entire Godhead… It is only the union of natures in the person of Christ and the work of salvation accomplished in the incarnate one that belong restrictively to the Son –and, of course, even those events are willed by the triune God.
Modern theologians who reject the principle of inseparable operation of the Trinity in all outward works often point to the incarnation as the gigantic exception to the rule. And if it were an exception, it would in fact seem to require a revision of the rule, since the incarnation of the Son is not just any old divine action. But it is not an exception. In the incarnation of the Son alone, the Son is not alone. Everything about the created reality that is incarnation was worked by the one God in three persons.
Much more can be said. Scott Swain recently drew attention to the importance and fruitfulness of the doctrine of inseparable operation; and of course Keith E. Johnson has come up from his deep dive into Augustine’s writings to write about it several times, notably in Themelios.
If you want to read a lot more about the baptism of Jesus in the Jordan and find much trinitarian meaning there, I wrote a multi-part exploration of the iconography of the event here.