Perichoresis could be regarded as a kind of theological black box. It has been used in the history of theology as a means of filling a conceptual gap in reflection upon the Trinity and the hypostatic union in the Incarnation. This gap has to do with how it is that the two natures of Christ, or the persons of the Trinity, can be said to be united in such an intimate way that, in the case of the Trinity, there are ‘not three gods, but one god’, and, in the case of the hypostatic union, that there are not two entities in one body, but two natures held together in perfect union in one person. Perichoresis fills this gap with the notion that the two natures of Christ and the persons of the Trinity somehow interpenetrate one another, yet without confusion of substance or commingling of natures. But what does it mean to say that the persons of the Trinity exist in perichoretic unity, mutually interpenetrating one another, or that the two natures of Christ subsist perichoretically, in a hypostatic union?

This chapter is an attempt to make some sense of these two applications of the doctrine of perichoresis to the Incarnation and Trinity.