I assume that mortal life is about learning, spiritual learning - that is, we have experiences, and therefore, if we make the right choices, opportunities to make spiritual progression towards divinity (i.e. theosis or sanctification).
I shall call this primary purpose of mortality divine-learning...
(The framework of which is that each us is incarnated into a personal situation - in time and space, with particular parents - where our lives have the greatest possibility of divine-leading to the experiences that we, personally, most need.)
But what does this 'divine-learning' mean? Well, what this learning is Not is learning in the everyday or scientific sense of observable 'behavioural-change' in mortal life. Because behavioural-change can't be what learning is about; because we humans are not designed that way, and neither is the world.
(We mortals are feeble, labile, distractible, prone to disease and sin etc. ; and our world is full of evil, temptations, sufferings and distractions (as well as love and creativity). Therefore, unless God is incompetent - which as creator he is not, then Christians (who acknowledge God the creator as wholly Good, and our Father) need to assume that this is (on the whole) the kind of world we need.)
Divine-learning - that learning from Life that you and I are living for - is about something much more than mere behavioural change; it is about a real, permanent... indeed eternal and spiritual change. The learning of our mortal life is designed to benefit our eternal life.
Divine-learning = Positive spiritually-progressing change that affects that which is eternal in us, lasting forever, beyond our mortal death.
Thus, when we (mortal incarnate Men) learn in this divine sense; it entails a change in reality.
It is repentance (a gift made possible by Jesus) that makes this learning possible.
(Before Jesus - repentance was not possible; without Jesus, repentance would not be possible - thanks to Jesus, repentance became always possible for everybody and anybody - including those who lived before Jesus.)
But what is repentance? - in this ultimate sense of divine-learning which goes far beyond observable mortal behavioural change?...
Repentance was a gift of Jesus - his incarnation, death and resurrection. By repentance, Jesus brought-in the change that from-now-on Men would not only learn passively and unconsciously (like young children)... but in the new dispensation that Christ initiated, our learning would be self-active, conscious, explicit to our-selves.
And this is repentance; repentance is actively learning from our mortal experiences, and knowing that we are learning, and knowing what we have learned. And this is what is permanent - going beyond the contingencies of the behaviours of our mortal lives.
Repentance = explicit and permanent learning from the experiences of mortal life.
