7. It is union with Christ, with his person. This goes beyond the indwelling of the Spirit in the church and its members, … (Robert Letham, Systematic Theology, p. 787)
This goes beyond communion. It entails union. It is more than participation in the communicable attributes of God. It is not to be restricted to union with righteousness, goodness, holiness, or truth, neither is it union with the benefits of Crist, as if it were union with the doctrine of sanctification. It is union with Christ. Moreover, the humanity of Christ were not simply united to some of God’s attributes; if it were, we would be left with an extreme form of Nestorianism. (p. 788)
Athanasius’s main term for expressing this is μέτοχοι (partakers). (p. 771)
The Eastern Orthodox teaching of theosis teaches that the godly will become partakers of the divine energies in what the western churches call glorification. Utilizing the essence/ energies distinction, theosis, while speaking of partaking of the divine, is not like fusing into the divine like a drop of water returns to the sea. Rather, it deals only with the "energies" of God. In Eastern Orthodoxy, that is basically stating that Man will become as close to the divine as creaturely possible.
The problem with theosis is that it remains an ontological category. This is compounded by Robert Letham who seems to embrace some version of theosis, which he insists is more than what the Reformed teaches about union with Christ. This is disturbing especially when he quotes Athanasius in his use of μέτοχοι (metachoi), as opposed to 2 Peter 1:4 where the term is κοινωνοι (koinōnoi). As seen in 2 Peter 1:4 therefore which deals with the divine, the use of koinōnoi rather than metachoi seems to indicate that the partaking is more of a perfect communion between two people, rather than any form of ontological unity.
It is for such reasons that Letham's embrace of some type of theosis is to be rejected. While not the same as ontological fusion, the ontological focus of theosis should make it unpalatable to all who hold to the Creator-creature distinction.