Theosis is the process of becoming more like God, more divine, more saintly (sanctified) during mortal life; theosis is the purpose of an extended mortal life (rather than simply being incarnated and then dying).
But this idea of becoming more divine tends to make theosis sound more rare and difficult than it really is; perhaps because we tend to regard it in a primarily moral way - so that the assumptions is that we are supposed to become more virtuous. Well, that is one type of theosis, the theosis of some great Saints: the saints of virtue.
But there are other kinds of Saints, and other kinds of theosis - for example of knowledge; specifically knowledge of truth, knowledge of reality - which is also a god-like attribute.
This fits ith the idea that extended mortal life is mostly about having experience and learning from it; every person, every place, every day is different - there is a lot to learn-from and a lot to learn... And we ourselves change, in both mind and body - for example we develop through childhood, mature, have emotions, get sick, age and die. We make decisions (good and bad) and live with the outcomes...
Mortal incarnate life therefore seems to be an excellent situation in which to learn; and perhaps/ presumably in this respect it is far superior to pre-mortal spirit life, and post-mortal resurrected life.
Theosis could be conceptualised, in a general and inclusive sense, as exactly this learning; we are meant-to learn from our life experiences; and when we do this is theosis - it brings us quantitatively closer to the divine level of knowledge as well as virtue.