I'm going to briefly sketch out what I understand to be a proper Reformed way to speak about theosis, mostly as a mental exercise and also for feedback. So, here goes:
Man is designed by nature to be the image and glory of God. He reveals, represents, and rules for God in creation. Every facet of his constitution in body and soul was crafted for this end. This is his final cause, toward which his whole self organically strains.
Imaging God is not a static state from the beginning but man's proper end. Man begins as the image of God, but incompletely developed, as a child begins as a man but not a mature man. The full glory of imaging God is to the whole man what physical adulthood is to the body.
A thread: In John 6, Jesus says, "All the Father gives me will come to me; and him who comes to me I will not cast out," and "No one can come to me unless the Father who sent me draws him." These verses take high place in a common argument that the passage teaches most of TULIP.
No one can come unless the Father draws? Total depravity.
The Father gives people to the Son before they can even come to Him? Unconditional election.
All of these given people will come? Irresistible grace.
Jesus will never cast them out? Perseverance of the saints.
In one sense, this passage certainly does provide material from which these points of Calvinism might be legitimately inferred. However, we should note that the context in John and the overall passage indicates a different primary referent.