Christianity centers on Jesus Christ, the sovereign, saving, and satisfying Son of God. It is good, therefore, for us to take time to think often about the person of Jesus. Who is he? Where does he come from? How is it that he is both fully God and fully man? In other words, even though it is right for us to think long and hard about the work of Jesus (his life, death, resurrection, and intercession), we should not pass over his person.
When we think about the person of Jesus, some amazing and complex issues are raised. How is it that in the one person of Jesus, there exists two natures (one divine and one human)? Or, how is that Jesus, being fully God, can shed his blood and die for sinful men and women? After all, God is incapable of dying! These are amazing questions and historically, Christians have provided some careful answers.
In his Institutes of Christian Religion, John Calvin takes up many of these questions and spills a decent amount of ink to explain how Jesus is both divine and human. One of the contributions that Calvin makes to the discussion of the personhood of Jesus is known in academic circles as the extra Calvinisticum. Fancy words, but what do they mean? I’ll quote the relevant portion of Calvin’s Institutes, then, with the help of Kevin DeYoung, offer a short explanation.
Here is the relevant portion from Calvin:
For although the boundless essence of the Word was united with human nature into one person, we have no idea of any enclosing. The Son of God descended miraculously from heaven, yet without abandoning heaven; was pleased to be conceived miraculously in the Virgin’s womb, to live on the earth, and hang upon the cross, and yet always filled the world as from the beginning.
The Institutes, Book 2. Chapter 13. Section 4.
Note carefully Calvin’s words. The eternal Word, or Logos, who has existed as the second person of the Godhead for all eternity, has “united with human nature.” That’s the language of incarnation. As John wrote, “the Word became flesh and dwelt among us” (John 1:14). Yet, even though divinity has taken on humanity, the divine is not restricted to a fleshly box (i.e. “we have no idea of any enclosing”). Instead, in some ineffable way, the divine Son of God, the eternal Word, dwells with mankind and, at the same time, fills both heaven and earth.
This, then, is the extra. As Kevin DeYoung explains, “In other words, the Son, even in his incarnate state, is able to live a divine life outside (extra) his human nature” (DeYoung, Theological Primer: The Extra). And why is this important? It reminds us that though Jesus has humbled himself by taking on the form of a servant (Phil 2:7), he has not ceased to sustain the world by the word of his power. DeYoung drives the practical point home.
The extra also reminds us that in the incarnation “the Son did not cease to be what he had always been” (Wellum, God the Son Incarnate, 332). He continued to sustain the universe (Col. 1:15-17; Heb. 1:1-3) and to exercise his divine attributes together with the Father and the Spirit. When Mary conceived a child by the power of the Holy Spirit, the divine nature did not undergo any essential change. Better to say the Person of the Son became incarnate than to say the divine nature took on human flesh (for the latter suggests the divine nature changed in its essential properties).
All this means–because the divine nature did not undergo essential change–that in coming to earth, the Son of God did not abdicate his rule, but extended it. It also means–because the human nature was not swallowed up by the divine–that the Son’s earthly obedience was free and voluntary. In short, the extra protects a Chalcedonian understanding of the incarnation that Christ’s divine and human natures were indissolubly joined, yet “without confusion” and “without change.”
Does all this sound complicated? Well, admittedly, it wrestles with some massive questions. But I cannot think of any better use of your God-given intellect than to spend time thinking about the person of Jesus Christ.
Theology Along the Way 