He had not lost His former being, but He had become what He was not before; He had not abdicated His own position, yet He had taken ours.

-Hilary of Poitiers, De Trinitate 3.16

Advent is well under way and Christmas is nearly upon us. So Christians around the world are giving special attention to the glorious mystery at the heart of our faith: the Incarnation of the Son of God for the salvation of the world. But what does it mean for God to become incarnate? How can a single individual be both God and man?

Even among those who agree upon the basic grammar of Christology enshrined in the Chalcedonian Definition–namely, that Christ is one person in two distinct but inseparable natures–there is often confusion and disagreement about what kind of incarnation we are talking about. What would it even mean for God to become incarnate? In what follows, I briefly map out some of the major options provided by contemporary theologians and philosophers in answer to this question (these two books have been especially helpful to me in thinking through these issues).

Kenosis vs. Krypsis

Many Christians assume that in order for God to become incarnate, he must surrender something–if not the possession of his divine attributes, at least the exercise of certain divine attributes (such as omnipotence or omniscience). After all, how can Christ be genuinely human, if at the same time he possesses all knowledge and all power? How can he be temporally and spatially located, if at the same time he is timelessly eternal and omnipresent?  It must be the case, on this reasoning, that the Son divested himself of certain divine attributes in order to become incarnate. This so-called kenotic theory of the incarnation–which takes its name from Phil. 2:7, he “emptied” (ekenosen) himself–remains popular among many Christians and has witnessed a revival of interest among academic theologians in recent years.

But the kenotic model is a relatively recent theory of the incarnation, with roots in 19th century German Lutheranism. Older interpretations of Philippians 2 understood the Son’s self-emptying, not as an actual divestiture of deity, but as a refusal to demand that his deity be recognized, if it should interfere with the divine purpose to save. It was, as Oliver Crisp has suggested, not so much an actual, metaphysical kenosis as it was a divine krypsis: a veiling of the Son’s forma Dei (form of God) under the guise of the forma servi (form of a servant). This understanding is consistent with the oft-cited Patristic formula that the Son of God became what he was not, without ceasing to be what he was (e.g., see the Hilary quote above). It is also consistent with the so-called extra Calvinisticum: the notion that the Son of God is not limited to nor circumscribed by his human nature, even in his incarnate state, but instead continues to live out, so to speak, his immutable divine life along with the Father and the Spirit.

Transformational vs. Relational Models

Closely related to this first pair of options is a second set of categories, namely, what Jonathan Hill has termed transformational vs. relational models of the incarnation. In the transformationalist approach, as one might guess, the Son is transformed into a human being either by becoming a human body or a human soul. The most problematic version of transformationalism can be found in the heresy of Apollinarianism, which maintained that the Son simply replaced the human soul in Jesus Christ. But kenotic models can also be seen as a subset of the transformationalist approach, since, in kenoticism, the Son has restricted himself, as it were, to the constraints of an ordinary human life.

In contrast, relational models understand the incarnation in terms of the Son acquiring a particular relation to a particular human nature. This human nature is complete (body and soul) and would have constituted a distinct human person had it not been assumed by the Son. But since it is assumed by the Son from the moment of conception is does not constitute a distinct human person (anhypostasia) but is given its personhood in the hypostasis of the Son (enhypostasia). On the relational account, then, the Son need not surrender anything, nor undergo any kind of transformation in terms of his divine life, in order to assume a discrete human nature.

Abstractism vs. Concretism

The final set of categories tracks closely with the previous two. This final typology concerns the question, what kind of human nature did Christ assume? Did he assume a concrete human nature–complete with both body and soul–as the relational model suggests? Or instead did he merely assume a set of abstract properties that are common to human nature?  In other words, did he assume a human nature, viewed as a concrete particular, or did he assume human nature, viewed as an abstract universal? Alvin Plantinga appears to be the first to suggest this terminology (abstract vs. concrete human nature) but the positions go back much further (perhaps even to Chalcedon, as Plantinga suggests). As the tradition of the undivided church continued to clarify and expound upon Chalcedon, it became apparent that the orthodox position demanded something much closer to concretism, since the sixth ecumenical council affirmed that Christ has not only two natures but also two wills and two energies.

Conclusion

These sets of categories are obviously related. The first factor in each is related to the others. In other words, kenoticism, transformationalism, and abstractism can be understood as distinct ways of describing the same basic approach to the incarnation. Likewise, krypsis, relationalism, and concretism appear to hang together as a coherent model of the incarnation. If we were to state these contemporary options in terms of ancient Christological heresies, we might say that the first approach is eager to avoid Nestorianism (the two persons heresy) but perhaps skirts too closely to Apollinarianism (an incomplete incarnation). The second approach would run in the other direction: keen to avoid Apollinarianism but running the risk of Nestorianism. For my own part, I think the second approach–the kryptic-relational-concretist model–is the most consistent with both Scripture and the Christian tradition; can successfully avoid the charges of Nestorianism; and is superior to the first model in terms of broader dogmatic considerations. But that is an argument for another day!