The Divine Exchange
In this manner Christ takes to himself our birth and absorbs it in his birth; he presents us with his birth so that we become pure and new in it, as if it were our own, so that every Christian might rejoice in this birth of Christ and glory in it no less than if he, too, like Christ, had been born bodily of Mary.
~Sermon the Gospel for Christmas Eve, Luke 2:1-14, LW 52:15.
The “Divine Exchange.”
That’s the theological name for what Luther is describing in this Christmas sermon. It comes from Roman law about marriage, where there were two ways of recognizing property rights. One recognized that someone owned a thing; the other recognized that one had entitlement to full use of a thing. When a couple was married, each simultaneously retained what each previously owned but now also had full rights to use what the other owned as if it was truly theirs.
An illustration: let’s say that when I was married a drove a base model VW Jetta, old enough to be on its last legs, and my wife owned a BMW as new as it was luxurious. While my wife still owned her BMW, I now had full use of that car whenever I needed it as if it was really mine, and whereas I still owned my Jetta, she – lucky girl! – had full right to use it anytime she wanted. (I should probably make clear that this is partly hypothetical. I did own an old, base model Jetta, but my wife did not own a BMW – too bad for me.)
According to Luther, that’s what happens in the Incarnation: God is joined to us fully and completely in the flesh of the birth of the Christ child and thereby takes on our life – our hopes and dreams, faults and sins, ups and downs – and gives us Christ’s own righteousness. The sins are ours, but Christ takes them on as if they were his; and while the righteousness is Christ’s, we now can make full use of it. Which means that when God looks at us, God only sees us as fully righteous and pure as Christ, kind of like those who saw me driving my wife’s hypothetical BMW as my own car.
That is, for Luther, one of things that makes Christmas so special: it is the celebration of God’s intent and action to be joined to us fully, to take on our lot and life completely, to give us all grace and goodness, and to promise always to see us and treat us as if we are Christ. Or as St. Athanasius said a millennium before Luther: “Christ became like us so that we might become like Christ.” Blessed Christmas indeed!