"only because of the uniqueness of the form of causality that is proper to God as Creator is he alone free to 'become' human, to assume a created nature hypostatically, without being in any way alienated from what he is eternally. The substrate of the hypostatic union is the existent person of God the Son. Therefore, if there is a hypostatic union, the causality entailed cannot transpire in a pre-existent material subject in which change is effectuated. Rather, this union is the new presence of God in creation existing as man, with a human soul and body. The subject is the Son. Just as creation does not effectuate a change in the creature, but gives existence to it, so too the Incarnation is not an intra-worldly change, but the gift of God existing in human flesh. Precisely because God alone can act at the level of existence in a causal fashion, therefore he alone can become incarnate in the being of man (at the deepest level of created reality) without ceasing to exist as God. It is truly God the Son (the author of life, in whom we live and move and have our being) who is present in history, yet without any loss of his deity. . . .
"This is a non-trivial matter, for if by contrast we remove the appeal to the analogy of creative causality from our understanding of the divine and human natures of Christ that are united in his person, then we must conceive of the union of God's divine and human natures not in a trans-historical fashion (aided by recourse to an analogical doctrine of creative causality), but rather by appeal to a likeness from causal becoming in a pre-existent subject. The hypostatic union of the natures will then have to be 'narrated' by a movement of the Son from being God alone into being human, understood after the fashion of the change from one specific state or contrary to another within a common genus, whether this be the genus of 'nature,' 'relation,' or 'operation.' The non-relativity (and non-mutual reciprocity) of the divine and human natures will be lost. Instead, God will be understood in a narrative fashion, through historical becoming, as one who is eternally relative in his deity to the human nature of Christ. Therefore, the Incarnation will not be conceivable without ascribing history to the very life of God in se, and the very notion of the 'immanent Trinity' will be threatened. . . . [And] if God is only intelligible in himself as the triune God in relation to the historical economy (which includes moral evil, suffering, and death), then these latter attributes of history are also in some real sense intrinsically necessary to the developing identity of the historical God. In this case, the results of the abandonment of a classical metaphysics of divine causality are in fact disastrous, not only for the speculative contemplation of the Trinitarian mystery itself, but also for our ethical and soteriological understanding of the agency of God."
Thomas Joseph White, O.P., "How Barth got Aquinas wrong: a reply to Archie J. Spencer on causality and Christocentrism," Nova et vetera: the English edition of the international theological journal 7, no. 1 (Winter 2009): 264-266 (241-269).