I only became aware of theosis myself relatively recently, so have no wish to promote myself as an expert. Some of the folk I read, however, seem to be, and universally indicate that divinization is on the symbolic level. The Christian doctrine of soteriology, that what Christ did not assume cannot be sacralized, seems to be based upon it.
They [the Cappodocians] took for granted that the attainment of likeness to God was the telos of human life. But God remains in his essence utterly beyond human grasp. The deification of the Christian is subordinated to this by being kept to the ethical and analogous levels.
Russell, Norman The Doctrine of Deification in the Greek Patristic Tradition [Oxford Early Christian Studies, 2004, p. 233]
As I understand things, we cannot become God himself, who is entirely Other. But the promise of assumption into the divine life through Christ's grace in the resurrection is only valid if Christ was both fully divine and fully human: if Christ was not True God by nature then we could only be assumed into a pseudo-divine eternity- not the real thing; and if he wasn't fully human then humans couldn't be fully assumed into the divine life in the resurrection. We are sons by adoption whereas Christ is Son by nature.
But the Christological role of theosis was gradually underplayed, as it has obvious interpretive problems concerning becoming "divinized" by grace. No one gets to suddenly perform miracles or become superior to their fellow man/sinners. Indeed, one is usually made aware of how humble one really should be. To seek superiority or "divinity" pretty much precludes genuine theosis,IMHO.
With Cyrils death in 444 deification disappears from view. In the search after Chalcedon to find a way to make the councils Definition acceptable to the monophysites, only Leontius of Jerusalem refers to it. Deification was perhaps felt to give too much away to Apollinarianism and Eutychianism. It reappears at the beginning of the sixth century with Dionysius the Areopagite, through whom an influence from the pagan Neoplatonist Proclus also enters into the Christian tradition, and is then used with great frequency by Maximus the Confessor in the seventh century. Maximus achievement was to reclaim deification for the Byzantine Church. He abandoned the Christological use of the term and developed it in a completely new way, making it central to his teaching for a monastic audience on the ascent of the soul. In doing so he built on the work of Cyril and the Cappadocians, drawing also on Evagrius Ponticus, the Macarian Homilies, Diadochus of Photice, and Dionysius the Areopagite.
Russell, Norman The Doctrine of Deification in the Greek Patristic Tradition [Oxford Early Christian Studies, 2004, p. 237]