The hypostatic union

Two natures, one person

To confess two natures in Christ raises the next question: how are they one? The church's answer is that they are united ‘according to the hypostasis’, that is, in the person. Hypostasis is the Greek word the church used for each of the three, the Father, the Son and the Spirit (see hypostasis); it is the word behind Hebrews 1:3, where the Son is the exact representation of God's nature (hupostasis (ὑπόστασις)). The hypostatic union means that the two natures are joined in one ‘who’, the eternal Son.

It is not God dwelling in a good man, as he inspired the prophets; nor two persons working in partnership; nor a blend of natures into some third thing. Cyril of Alexandria wrote to Nestorius that the Word united to himself, according to hypostasis, flesh animated by a rational soul, and so became man Cyril, Second Letter to Nestorius. The one born of Mary and crucified under Pilate is the Son of God himself. His human nature never existed as a separate person; from the moment of conception it has existed in the person of the Son. The union was not broken even by death, and it will never end. The Athanasian Creed offers a comparison:

For as the reasonable soul and flesh is one man; so God and Man is one Christ

The Athanasian Creed, clause 34

What belongs to one belongs to him

Because the union is in one person, Scripture can say of him, named by one nature, what belongs to the other. Paul urged the Ephesian elders:

Be shepherds of the church of God, which He purchased with His own blood.

Acts 20:28

Blood belongs to the human nature, but the one who shed it is the Son who is God. The rulers of this age "crucified the Lord of glory" (1 Corinthians 2:8). The church calls this the communication of properties (communicatio idiomatum). The Westminster Confession says that "by reason of the unity of the person, that which is proper to one nature is sometimes, in Scripture, attributed to the person denominated by the other nature" Westminster Confession 8.7. The title Theotokos for Mary follows the same logic. God did not die as God; the one who is God died in the humanity he made his own. That is why his death, the death of a man, has infinite worth.

Where Lutherans and the Reformed differ

Both confess Chalcedon, and both reject Nestorianism and Eutyches alike. They differ over how far the human nature shares in divine properties because of the union. The Lutheran Formula of Concord (1577) teaches that the exalted Christ shares God's power and majesty also in his human nature, so that he can be present bodily where he has promised, above all in the Lord's Supper Formula of Concord, art. 8. Only so, Lutherans hold, is Christ not divided.

The Reformed teach that in the union the human nature remains a finite, true human nature. Calvin wrote that the Son was borne in the virgin's womb without leaving heaven, and went on filling the world as he had from the beginning Calvin, Institutes 2.13.4; Lutheran theologians called this the ‘extra Calvinisticum’. The Heidelberg Catechism answers the charge that this separates the natures:

since the Godhead is incomprehensible and every where present, it must follow that it is indeed beyond the bounds of the Manhood which it has assumed, but is yet none the less in the same also, and remains personally united to it

Heidelberg Catechism, Q. 48

Lutherans fear the Reformed account pulls the natures apart; the Reformed fear the Lutheran account blurs the true humanity of Christ. Both mean to keep the confession of Chalcedon. See also the lesson Chalcedon: one person, two natures and Who is Jesus?