The Immutable Son and His Incarnation

The word “incarnation” is a construct of Latin words for “on” (in) and “flesh” (caro). Most immediately, the English word derives from the older French word incarnacion. In the Greek of the central passage in John 1:14, it is just the common word that we see Paul always using for “flesh” and even “sinful nature” (sarx). So the context has to decide the word’s exact meaning. In the case of the Son of God “becoming” flesh, it is perfectly natural to start thinking about the implications for how the divine nature and human nature relate.

This is crucially important when our mind makes its way over to the prospects of change. However, the Incarnation implies no change whatsoever to the divine person or nature. To avoid any confusion, theologians will speak of the Son “assuming to himself” a human nature. As Athanasius famously put it, “He became like us that we might become like him.” But of course, “become” is a manner of speaking, whether it was an early church father doing the speaking, or John himself, inspired by the Holy Spirit. 

What happened to the Trinity during the Incarnation?

The short answer is that nothing “happened to” the Trinity “at” the Incarnation. There was no absence in heaven “during” the Incarnation (nor was there any “during,” incidentally), as the divine nature of the Son was not and could not be diminished. All texts referring to the sending of the Son, in terms either the spatio-temporal or the mutable, signify changes outside of the divine essence and in his human nature.

Moreover all statements about the union of the two, such as in “the Word was made flesh” (Jn. 1:14) or “God with us” (Mat. 1:23), are possible because of a concept that theologians call the communicatio idiomatum. This is the Latin term for the “communication” or “sharing” of the properties.

Different traditions will debate how to apply this to various issues, but at its core it implies that (1) whatever is true of either nature (divine or human) may be said, in a real sense, of the whole Person of the Son, so long as (2) each nature remains unconfused with the other. A biblical example is Acts 20:28. God does not bleed, yet Paul speaks of God purchasing the church “with his own blood.” Clearly this refers to the Son’s blood in His human nature, yet Paul could legitimately predicate this blood to “God” as the proper subject. 

But how is it that the Son does not change states in the Incarnation?

Two doctrinal truths and one philosophical distinction will help us see why the incarnation implies no change in God.  

The first truth is that Christ’s humanity is created and is thus mutable. That his divine essence is immutable must be kept fixed and yet distinct in our thinking. The second truth follows divine immutability, namely that all change arising from any creation belongs properly to the work outside of God (ad extra) rather than to anything in the divine essence (in se). 

To all of the objections that have long been leveled against that doctrine, a particular distinction of modern philosophy is useful, that of so-called “Cambridge properties” and “Cambridge changes” (so named because the philosophers in question taught there). It is actually a very simple concept. If my youngest son outgrows his older sister, he will become taller than her. Conversely, she has now “become,” by way of relation, shorter than him. But she need not have shrunk to have experienced such a change. All of the change occurred outside of her, and is really a manner of speaking. Don’t overextend the analogy. Here the older sister really does “experience” a kind of change and so, to that extent, is a creature of impression. The analogy only applies to the logical relation aspect.

Now in putting all of this together, when we say that the Word “became” flesh, this too is a manner of speaking. Classical theologians would even use the language that he “took on,” “united himself to,” or simply “assumed,” flesh. They were well aware that the Bible itself uses the language of “becoming” (again, John 1:14), but the point is to emphasize that this is really phenomenological speech, and not meant to be an ontological assessment of change in the eternal Word. 

Stephen Charnock’s section on immutability in his Discourses on the Existence and Attributes of God (1680) is especially profound on this point. As he put it, “There was no change in the divine nature of the Son, when he assumed human nature;”1 that is, “by assuming or by acting, not by being acted upon.” On the contrary, he wrote, “there was an union between the two natures, but no change of the Deity into the humanity, or of the humanity into the Deity.” As to Paul’s meaning in Philippians 2:7, of the Son’s emptying himself, Charnock added:

“The glory of his divinity was not extinguished nor diminished, though it was obscured or darkened, under the veil of our infirmities; but there was no more change in the hiding of it, than there is in the body of the sun when it is shadowed by the interposition of a cloud.”2

Augustine once gave another analogy which really gets to the heart of what is meant by this kind of becoming:

“And just as our word becomes sound without being changed into sound, so the Word of God became flesh, but it is unthinkable that it should have been changed into flesh. It is by assuming it, not by being consumed by it, that both our word becomes sound, and that Word became flesh.”3 

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1. Stephen Charnock. Discourses on the Existence and Attributes of God, I.339.

2. Charnock. Discourses, I.339.

3. Augustine, On the Trinity, XV.3.11,20.