From time to time I make new entries in this continuing series called “Theological Primer.” The idea is to present big theological concepts in around 500 words. Today we look at the doctrine of perichoresis.
It is a recurring theme from the lips of Jesus that the Father dwells in the Son, that “I am in the Father and the Father is in me” (John 14:10-11). All that Jesus asks in the high priestly prayer is rooted in the reality that the Son is in the Father, and the Father is in the Son. The apostle Paul, likewise, testifies that in the incarnate Son “all the fullness of God was pleased to dwell” (Col. 1:19).
We usually understand these verses to be about Christ’s deity. And rightly so. But they also speak to the mutual indwelling of the persons of the Trinity. The Father, Son, and Holy Spirit are distinct persons—distinguished, respectively, by paternity, filiation, and spiration. And yet, we must not think of the three persons as three faces in a yearbook. The Father indwells the Son; the Son indwells the Spirit; the Spirit indwells the Father (and you could reverse the order in each pair).
The Greek term used to describe the eternal mutual indwelling of the persons of the Trinity is perichoresis (in Latin, circumincession). The word circulatio is also sometimes used as a way of metaphorically describing the unceasing circulation of the divine essence, such that each person is in the other two, while the others are in each one. At the risk of putting things in physical terms, perichoresis means that “all three persons occupy the same divine ‘space.'” In other words, we cannot see God without seeing all three persons at the same time.
The mutual indwelling of perichoresis means two things. First, the three persons of the Trinity are all fully in one another. And second, each person of the Trinity is in full possession of the divine essence. To be sure, the Father is not the Son, the Son is not the Spirit, and the Spirit is not the Father. Perichoresis does not deny any of this. What perichoresis maintains is that you cannot have one person of the Trinity without having the other two, and you cannot have any person of the Trinity without having the fullness of God. The inter-communion of the persons is reciprocal, and their operations are inseparable. As Augustine put it: “Each are in each, and all in each, and each in all, and all are one.”
Like many aspects of Trinitarian theology, this one can be hard to grasp; we have to rely on careful verbal definitions rather than concrete analogies. We must not think of perichoresis—as some have suggested from the etymology of the word—as a kind of Trinitarian dance. Such an analogy, and its social Trinitarian implications, undermines the truth that perichoresis means to protect. Here’s the problem: How can three persons simultaneously share the same undivided essence? The answer is not that Father, Son, and Holy Spirit waltz in step with each other, but that they coinhere in such a way that the persons are always and forever with and in one another, yet without merging, blending, or confusion. Only by affirming the mutual indwelling of each in each other, can we worship our triune God as truly three and truly one.
 Gerald Bray, Doctrine of God, 158.
 Augustine, On the Trinity, 6.10.