On the list of doctrines that Witherington took ever so subtle jabs at (so subtle, in fact, that the person sitting next to me in the lecture did not even remember it) was "theosis." I put that in quotes because the doctrine that Witherington opposed was nothing like the doctrine of theosis espoused by the Orthodox Church. Reading 2 Peter 1, Witherington scoffed at those who held to a view of "theosis," people who thought that we would become omniscient, omnipresent, and so on.
Frankly, I can understand why that concept of theosis would be unnerving for Witherington. After all, the Orthodox Church for centuries has worked deliberately to resist any understanding of theosis which would equate divinization with becoming God in any absolute sense. Witherington inexcusably confused taking on divine characteristics as becoming divine absolutes. The Orthodox doctrine of theosis does not allow for people to appropriate the absolute characteristics of God. In fact, it positively precludes it.
The doctrine of theosis is one of perpetual ascent, the expansion of finite beings into an infinite plenitude of God. It doesn't matter to what degree a finite reality grows, it can never fill infinity. The point at which a being reaches omniscience, for example, is the moment at which knowledge ceases to be finite and becomes infinite. Unless you view God's omniscience as merely His comprehensive knowledge (in some way analogous if not identical to our own mode of knowing) of things extant rather than as His infinite capacity to contain everything as cause, there is no reason to assume that without a dramatic ontological shift that the finite human should ever expand to a point where he has reached complete knowledge. Complete knowledge is not something absent in humanity merely from some lack of information. Absolute knowledge requires an absolute being who transcends knowledge and knowing as cause.
Theosis never allows for humanity to make that transcendent leap from finitude to infinitude. In fact, the idea of perpetual ascent absolutely excludes the possibility that we should ever arrive at an absolute. To be saved as creatures, we must remain creatures even as we are united with divinity. Notice the language is of "union" and not of "absorption." Perhaps Witherington had in view the doctrines of the Latter Day Saints, in which people themselves become deities, or of certain forms of Buddhism, in which people are absorbed into a universal Nothingness. That is not, however, "theosis" properly speaking. Theosis has its type in the Incarnate Word: a union of two natures without confusion, in which both retain their essential characteristics. (This Christological aspect of theosis is a problem to be dealt with later.)
Witherington in his brief and subtle critiques of theosis only managed to display and undo his own misconception of the doctrine. Theosis has always rejected the confusion of deity with humanity. It has nothing to fear from accusations that it will lead to omniscience, omnipresence, or really omni-anything in the way that God possesses those qualities.