I do believe that the concept of theosis, or whatever word we want to use to describe what it stands for, is part of the foundation of all Orthodox spirituality and it belongs to all those, whatever the Orthodox community to which they belong, as part of a common inheritance. Of course that is not enough. It is just a word. We need to be very clear what we mean by this word and what we do not mean.
What it does not mean, according to any Orthodox writer I am aware of, is that we become divine according to our own nature so that we cease to be human. We do not become uncreated so that we cease to be creatures. We do not and never could become all powerful, everywhere present and without beginning. The Virgin Mary, the Theotokos, has not become one of the Holy Trinity and nor will any of us. We do not and could not ever become divine in the manner in which we think of the nature of God. My own view is that it is quite clear that in eternity, the faithful who enter the joy of eternity will have a body of some sort, and will not be a disembodied spirit. There will also be a new universe, renewed and recreated, in which the life of eternity will be experienced.
None of this is how we describe God. God is a spirit. He is not created. He does not live inside any sort of universe. He is beyond all being and his essence is unknown to us. If we were able to become God in the way that the Holy Trinity is God then since God does not experience time or the limitation of the created universe, and exists outside time, so we would also, being God in this way, be outside time, and so we would have to say that we had always been God, and if we had always been God then there would not be a Holy Trinity, but a God who is millions upon millions of persons.
We do not believe this. This is not what is meant by theosis.
But theosis equally does not mean simply trying to live a good and moral life so that we please God. It is not another way of just saying holy. It does not mean that we cease to be human, but it equally does not mean the sort of good behaviour we can manage to produce on our own.
We can take the example of the moon to illustrate what theosis means. Often the light of the moon, especially a full moon, is so bright at night that we are even able to read by its light. Yet we do not mean that the moon has turned into a source of light, such as the Sun, and that is shines by its own power. Nor do we mean that without becoming a star or Sun, it is shining by its own effort and trying to be like the Sun while only being barren rock. In fact we know that if the Sun ceased to exist then the moon would be entirely dark. The light of the moon is entirely the reflected light of the Sun, which transforms the moon into that which is beyond itself while it remains what it is. It remains a dead ball of rock, but by the light of the Sun it become a source of light without being changed into something else.
“And gazing at him, all who sat in the council saw that his face was like the face of an angel”.