Webster on the hypostatic union
Like the notion of substance, the Chalcedonian dogma of the hypostatic union (the union of the divine and human natures of Christ in one person) is not a replacement for Jesus’ history, but a means of identifying the unique subject and agent of that history. It is thus all-important to note that the centre of gravity of the definition is not the natures considered in isolation from each other, but rather the event of their union in the one subject Jesus. The definition starts from the union of the natures in the one person and hypostasis of which they are predicated, rather than trying to construct a psychologically credible person by first defining and then uniting two distinct natures. Abstract exposition of the natures is excluded; this subject and agent is irreducibly at the centre. ...—John Webster, Word and Church: Essays in Christian Dogmatics (Edinburgh: T&T Clark, 2001), 145-47.
The union is utterly unique, an instance of itself, and in no sense a complement, completion or parallel to any other realities. The hypostatic union is not the most exalted instance of human self-transcendence, of humanity’s being ‘in God’ or of the immanence of God in creation. The incarnation cannot be traced either on the trajectory of humanity’s capacity to transcend itself and lose itself in God, or on the trajectory of God’s indwelling of creaturely reality. These trajectories may or may not have theological validity; but the hypostatic union is categorically different, neither generalizable nor the condensation of more general realities. Like the relations between the persons of the Trinity, the relation of divinity and humanity in the incarnate one defies analogies. The ground for affirming this uniqueness is the fact that the hypostatic union is an utterly free, uncaused and wholly underiveable act of divine omnipotence, and not merely the culmination of a series or the intensification of an ontological principle of wide application. ...
The dogma is distinctly a posteriori, a description of an act, not a speculative arrangement of a priori considerations. All along the line, therefore, grasping the dogma is a matter of deploying concepts in such a way that they display a singular reality which precedes and transcends them. Once again: person precedes natures.