Theosis (at root the same notion as sanctification, exaltation or deification) is the general idea that throughout our earthly mortal lives we are supposed to become (in some way) more like God, or perhaps more like "a god".
"Supposed to" because this is why we are sustained alive -- After all, why stay-alive (in the past and now) rather than simply dying and achieving salvation as soon as we choose to follow Jesus Christ to eternal life?
(Because that would surely be a more certain salvation: To die at the split second we converted, at the instant we made a commitment to follow Jesus. There must therefore be a very important reason why it does not happen.)
But theosis is difficult to conceptualize except in the rare instance of the greatest Saints; who have very obviously become more divine throughout their mortal lives (head in Heaven, feet still upon earth - as the Eastern Orthodox say).
It has often, and truly, been observed that becoming a Christian does not (or only seldom) make somebody overall a better person - so that, if theosis is indeed an integral aspect of genuinely Christian living, then the process doesn't seem to work very well...
I have, therefore, found it difficult to explain to myself - in some kind of comprehensible 'model' - what is supposedly going-on with theosis - but I now think I may have found a useful picture of the process, as it is intended to operate.
My assumption is that we have a primal self - which could also be called our real, true or divine self; and it is this which is eternal, and has existed from eternity. My primal self is "encased" within a mortal and temporarily-incarnated self; which is (approximately) our body and our personality - that which other people observe, and which interacts with The World.
The process called theosis describes the transformation of my primal self, across a timescale of eternity; but at present intended to be achieved by interaction-with, and learning-from, the experiences of my mortal self in this world.
So -- if I succeed in my God-given task of learning from the experiences God has set-up for me in this mortal world; then it is my primal self that is positively-transformed by this learning.
And it is this process of positive transformation of the primal self that can be called theosis.
This model may explain why it is that theosis is not necessarily (or usually) observable in a Christian individual.
What is happening is that the primal self is being-transformed positively and eternally - but the bodily behaviour and actions, and personality level motivations and thoughts; are Not (or not usually) being transformed.
So the primal self is getting-better when we learn Godly-lessons from our life experiences - whether or not the mortal self improves... or even gets worse!
This depiction maybe explains why and how it is that we may know someone who we are convinced has a Good Heart (i.e. the primal self); despite that his behaviour is clearly sinful and not improving; or exhibits grossly inconsistent, incoherent or chaotic behaviour.
And, on the other side; why it often seems (to our intuitive inference) that someone who leads "a Christian life", who seems to think and do the Right Things, who is nice, socially responsible, devout, a good neighbour etc.; may strike us as heartless, cold, unloving - and certainly Not improving as a result of his continued-living.
Or why we perhaps are sure that we our-selves are being made better by being-a-Christian; despite that we continue to sin in the same ways as much as ever, or in new ways, or backslide repeatedly - or even behave (to an external observer) overall worse than we did before becoming a Christian.
Another aspect of this mismatch between primal self and mortal self, is that it becomes understandable why God would allow (or even want) such a divergence.
The reason why we are sustained alive is to challenge us with repeated and multiple interactions with this world: experiences that are intended as learning opportunities.
And this situation may be easier to arrange if our mortal selves are Not (or not much, or only unevenly) positively transformed by life.
After all; the ultimate value of this mortal life is not within this temporary world, where nothing lasts and everything dies; its ultimate value is found in Heavenly life everlasting.
