Theosis is so critical a doctrine in Orthodox theology that it is essentially impossible to escape it in any aspect of Orthodox thought. It relates to soteriology (obviously), epistemology, theology proper, anthropology, and, among countless others, Christology. In this final aspect, I think that once again Witherington would have profited from a fuller, better understanding of theosis. During a very informal Q&A, Witherington made the assertion that the human nature in Christ was identical for all intents and purposes to the unfallen nature of Adam. (I mention the venue in the hopes that it may merely have been that Witherington, without having prepared, misspoke or spoke imprecisely.) This creates a theological quandary: would human nature inexorably but unconfusedly united to divine nature really have not been any different than human nature not so united? In the Orthodox tradition, the divine nature divinized the human nature, not in a way which was supernatural (that is to say, not in a way that exceeded the potentiality of any other human nature) but in a way that made that humanity something functionally other than all other human nature. It is only by being somehow different, not merely by virtue of the volitional activity of the human nature in Christ, that Christ can truly represent the new man, the true humanity.
I think the appropriate language to speak about the difference between Adam’s nature and the human nature of Christ, other than merely that one was united to divinity (which is theosis) and the other was not, is the language of potentiality and actuality. Adam always had the potential for increase, for union, for participation in the divine life, but that participation and its inevitable consequences had not occurred. He elected sin instead. Certainly, that participation in and union with divinity would have resulted in a radical change in him. Panagiotes Nellas has said that this potential was never truly possible before the Incarnation and argues that, even if there were no Fall, the Incarnation would have been necessary as a means of opening the path for union between God and man. Regardless of the merit of that argument, it was never beyond Adam’s nature to be in union with God. In that sense, the human nature of Christ and Adam were identical in their potential. After all, the human nature of Christ was truly human and no more. The suggestion, however, that they were no different in any respect seems to suggest (as I have accused Witherington of suggesting before) that the encounter with the divine can somehow leave all or part of human nature unaffected. I cannot accept this.
I dare say that a Christology which views the humanity in Christ as unremarkable misses something of the promise which we have in salvation. We are being transformed into something remarkable by our participation in the divine life. Christ, as the living embodiment of that participation, was already something remarkable even in his human nature.