Since the incarnation, Christ has possessed two complete natures, fully incorporated into his being. He is not a half-man, half god hybrid. He is 100% human and 100% God. His deity is infinite, and was never lost – even when he walked upon earth as a human. His humanity began at his incarnation but it too is eternal. He will never cease to be our human savior. This union of the two natures, or substances, is referred to as the hypostatic union. The term comes from the Greek word hupostasis, meaning substance.
The author of Hebrews uses the term hupostasis to express how Christ is the exact imprint of the Father’s nature. The ESV study Bible explains “Thus the Son is identical in substance to God, being himself fully God. In all attributes and abilities, the Son is exactly like the Father.” Already this work has shown that Christ possesses both deity and humanity. It remains to explain why that is necessary.
It was God’s plan from the beginning that the eternal Logos would become a human being and dwell here on earth with other human beings. He came down among us so that we could see his glory, a glory that only he and the Father share. He became one of us because that was the way to the cross. As Paul put it “though he was in the form of God, (he) did not count equality with God a thing to be grasped, but made himself nothing, taking the form of a servant, being born in the likeness of men. And being found in human form, he humbled himself by becoming obedient to the point of death, even death on a cross.” God’s plan required that the Logos retain his full deity while also becoming fully human.
Humanity needed a redeemer who was one of us, “yet without sin.” We were in a catch-22 situation. We could be redeemed from sin only by a sacrifice who identified entirely with our species. The sacrifice had to be human. But the catch was that our entire species had been defiled by original sin. Paul told the Romans that “all have sinned and fall short of the glory of God.” What we needed was both necessary for us to provide ourselves, and impossible for us to provide for ourselves.
God stepped in with his grace, and with himself. The Son of God became a Son of Man. With that one step of grace, it became possible once again for humanity to receive eternal life.
Not a Third Thing
Some have suggested that Christ was actually a fusion of God’s Spirit (the Logos) with human flesh (Jesus). They imagine that Jesus was a tertium quid (Latin for third thing). The Christian church has argued against this idea. One example of this idea was Eutychianism. This view held that Jesus’ human nature was overwhelmed by that of the divine Logos, and the result was a divine being who was different in nature from the Father. In its attempt to preserve the distinction between Christ and the Father, it denied what the Bible says about both.
Not Two Different Persons
Others, seeking to preserve the similarity between Christ and the Father, suggested that the human Jesus and the divine Logos were two separate beings. This idea is attributed to the Nestorians. Again, historically, such ideas have been rejected by Christianity because they do not fit the biblical facts. If Christ were two separate persons, then the human half could not have been sinless enough to die for our sins.
The significance of the Christ’s two natures in his one being cannot be overstated. When the eternal Logos became flesh he added humanity to his divinity permanently. This was an act of divine grace, and shows how valuable humanity is to our creator. God so loved the world that he divested himself of the prerogatives of his divinity – though still retaining his divine nature. He became obedient even to the point of taking on mortality, though he did not deserve it. He embraced mortality and the cross for us.
There are no human analogies that could explain exactly what the eternal Logos did at the incarnation. Perhaps one that comes closest is a physician who infects herself with a disease in order to cure the disease. Humanity itself was a disease, and it had infected the planet. Christ humbled himself to become one of us in order to affect the restoration and healing that was needed. So Paul says that “For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God.”
Having Christ’s Mind
Paul gives his explanation of Christ’s becoming human in the context of encouraging the Church at Philippi to become more Christ-like. He tells them…
Let each of you look not only to his own interests, but also to the interests of others. Have this mind among yourselves, which is yours in Christ Jesus, who, though he was in the form of God, did not count equality with God a thing to be grasped, but made himself nothing, taking the form of a servant, being born in the likeness of men. And being found in human form, he humbled himself by becoming obedient to the point of death, even death on a cross. Therefore God has highly exalted him and bestowed on him the name that is above every name, so that at the name of Jesus every knee should bow, in heaven and on earth and under the earth, and every tongue confess that Jesus Christ is Lord, to the glory of God the Father. Therefore, my beloved, as you have always obeyed, so now, not only as in my presence but much more in my absence, work out your own salvation with fear and trembling, for it is God who works in you, both to will and to work for his good pleasure. Do all things without grumbling or questioning, that you may be blameless and innocent, children of God without blemish in the midst of a crooked and twisted generation, among whom you shine as lights in the world.
The mind that Christ had is the one that believers can have. It is a mind that cares more about helping others than about looking out for number one. It is a mind that is willing to sacrifice what one deserves so that others can get the grace that they do not deserve. Paul says that this mind is ours in Christ Jesus. It is that mind, and the selfless actions it produces, that will lead this fallen world back to its creator.
 Hebrews 1:3.
 ESV Study Bible, electronic edition (Heb.1:3).
 John 1:14.
 Philippians 2:6-8.
 Hebrews 4:15.
 Romans 3:23.
 See http://en.wikipedia.org/wiki/Eutychianism
 See http://en.wikipedia.org/wiki/Nestorianism
 2 Corinthians 5:21.
 Philippians 2:4-15.