Theosis is the 'process' by which Men may become more divine. (Similar terms include divinisation and sanctification.)
For me, theosis is the primary purpose of mortal life - i.e of life extended beyond mere incarnation - whether that be to the stage of foetus, embryo, baby, child, adolescent, adult of into old age. In other words, I believe that God sustains our life (beyond mere incarnation) in order that we may have experiences and learn from them such tha we become more divine in this mortal life.
Albeit this 'being more divine' is (at most) a temporary experience; something we cannot 'hold-on-to during the changes of mortal life. Nonetheless, if we assume that learning has a spiritual and eternal dimension, and is not merely a matter of brain-sustained-memory, then even brief experiences of more-divine states may have a permanent effect on our-selves.
(If you ask why we cannot, in practice, hold-fast to anything in mortal life - or cannot be sure of doing so - then I would answer that this is because it is about experiences. We are not supposed-to make eternally-binding decisions in mortal life; because the proper time to do so is after death when it comes to a choice between resurrection in Heaven or other alternatives. Our experiences in mortal life are intended to aid that final, post-mortal decision which may be eternal; and eternally-binding.)
I would say that - with the qualified exception of the monastic type of Eastern Orthodoxy - theosis has never been a central goal in mainstream Christian life.
And the reason is obvious. It is that the 'gap' between Man and God is asserted to be infinite and qualitative - therefore there can be no-such-thing as movement-towards becoming 'more' divine. This because (according to mainstream, classical theology) we are creatures but God is uncreated; we are finite but God is infinite (omni-potent/-scient-/present etc.) The gulf is un-bridge-able.
This would seem to make it impossible for man to progress spiritually and become 'more divine' - whether that progression be gradual or incremental.
The description of Jesus Christ as having a dual-nature of both God and Man is of no practical help in explaining theosis; because in the first place it does not make literal sense but is a mystical formulation, while in the second place it merely kicks the can further down the road.
The dual concept of Christ has his divine nature as being the-same-as the divine nature of the Father and Holy Ghost, thus with all the infinite and omni attributes (infinitely-remote-from Man); and furthermore Men are Not part of that uncreated divine trinity, therefore Men cannot (presumably) have any share in real divinity.
Therefore, for theosis to be comprehensible and explicable in ordinary language (without recourse to non-coherent mystical word-formulations), entails that classical and mainstream theology be rejected and Man and God (and Jesus Christ) be seen as of the same basic kind (and presumably having the same ultimate origin), such that the difference between Man and God is quantitative rather than qualitative.
If so; then Men can become God bit-by-bit; by a spiritual progression - happening through time.
And theosis can then be seen as a divinely-intended goal of our mortal (as well as pre- and post-mortal) life.