Steven Paulson addresses this question in a densely-argued chapter of his book Lutheran Theology (see previous post). He begins by distinguishing the argument he is about to make from “theories of atonement,” which (he argues) try to fit the cross of Christ into the “legal scheme” of desert and punishment:
According to the legal scheme, sin is either a lack (debt) that must be compensated before the law can be satisfied (fulfilled), or sin is crime that must be punished. When Christ himself is pushed into the legal scheme its practitioners demand Christ make a payment for debt, absorb punishment, or provide compensation to those deprived of their goods (like the devil, the law, or even God himself) if he is going to serve as a true mediator between God and sinners. (p.91)
It is true, Paulson continues, that “Christ pays debt, suffers punishment, and pays ransom to the old lords of this world,” but he does so to destroy the legal scheme, not to vindicate it, and to create “an entirely new kingdom where the law has no service to render, no claim to make, and no more accusations against sinners.”
How does this happen? First we need to look at the two things that happen to redeem and reconcile sinners:
First, Christ became flesh and died once and for the cross, never to be repeated. Second, the preacher delivers the benefit of the cross by declaring the promise of forgiveness to sinners on account of that cross – repeatedly. […] There is a communication that occurred first in Christ’s own person between Creator and creature, divine and human, that reverberates through the preacher to communicate God-in-flesh to his forgiven sinners, including them in the new, free, life of God’s favour. (p.94)
This concept of “communication” between God and sinners is fundamental to Paulson’s argument, and takes him into the very depths of the incarnation itself: in particular, the communicatio idiomatum, the “communication of attributes,” that lies at the heart of Lutheran Christology (and which is drawn, as Martin Chemnitz sought to demonstrate, in the teachings of church fathers such as “Athanasius, Nazianzus, and especially Cyril”).
What is the communicatio idiomatum? Paulson summarises it as follows:
The communicatio idiomatum holds that there are characteristics or identifying features of the essence of a human creature on one hand (like being born, sleeping, crying, sinning, and dying), and of God’s essence on the other (like having no beginning, not sleeping, not crying, not sinning, not dying). Accordingly, Creatures are segregated from their Creator by these opposite “attributes” […] But in Christ incarnate, there is a new communication that effects exchange between creatures and Creator – expressed verbally in the scandalous language in which Luther luxuriated, such as: “God was born of Mary and lay in a manger,” and “the human Jesus created the world, and rules as Lord of the new kingdom.” (p.97)
More specifically, the communicatio idiomatum allowed Luther to insist that “the Son of God truly is crucified for us, that is, this person who is God” – in contrast to other Reformation figures such as Zwingli, for whom such language was anathema, and who insisted that only the “human nature” of Christ could actually suffer and die.
So the communication of attributes between the divine and human natures of Christ allows us to make such shocking statements as that found in a 17th century hymn quoted by Paulson: “O great dread / God himself is dead! / He died upon the cross…” But it also provides a basis for the “communication” between Christ and sinners that secures our redemption. On the cross, Christ assumes the sins of the whole world, taking them into his own body “actually, historically, physically” (see 1 Peter 2:24). As Paulson says:
[Jesus] is not assuming abstract, bookkeeping, non-historical, or impersonal sins. When Scripture says he takes the sin of the world, it means that eventually he gets round to taking your own personal sins. (p.106)
Paulson concludes his argument (of which I’ve barely scratched the surface in this post) with the following:
The sins of the world were laid on Christ; we communicated them to Christ by means of rejection of his words; what he communicates in return to sinners is unlike anything we have known: it is grace that is free and that creates a new world out of nothing – the law and sin are left behind forever because they have created nothing. […] The communication of attributes in the cross ceases the old segregation of the wrathful God and sinners. (pp.111f.)
Thus what are, under the law, impossibly separate are brought together in the gospel thanks to this “communication of attributes”: God and human nature in Christ; Christ and sinners on the cross; and (to bring this down, concretely, to earth) bread and wine and the body and blood of Christ in the Sacrament of the Altar.