It was seemingly difficult for Owen Barfield to express clearly what he meant by Final Participation of human consciousness - indeed I think he exhibited a reluctance to be explicit on this point.
I now feel I have sufficiently understood Final Participation to re-explain it in my own words; but in doing so I take a step further than Barfield was willing to go in most public fora; and I think I can understand why.
To make Final Participation clear involves acknowledging its basis in Christianity - which has a tendency to alienate non-Christians; while at the same time claiming to move-forward-from, and in that sense 'supercede' Historical Christianity - which would tend to alienate most Christians: thereby leaving Barfield with only a very small audience!
Anyway, whether or not the above understanding is a correct guess: here is my understanding of the assumed historical sequence of Original Participation - going through various phases to our current almost wholly-alienated Modern Western Consciousness Soul - to Final Participation.
The key concept is theosis, which is the process of becoming divine. The consciousness of theosis therefore clearly depends on the concept of the divine: in becoming like-god it depends what we understand by god.
Original Participation was the situation of the first Men - who lived in hunter gatherer societies. They understood the divine to be something like energies in a process of circulation and transformation. Theosis was therefore the living daily experience of participating in these energies and transformations. The system was closed, all is as it was and ever will be. Man is part of the divine, but not a separate self.
This was the childhood of Man.
Then came the start of an increasing degree of self-consciousness, of Man as aware of Himself as an Agent with 'free will'; which brought with it an increasing sense of separation from the divine. At first the separation was only temporary and could be overcome by the activities of priest, performing rituals, in temples - and the ultimate aim was to restore each man into the divine. Mundane life was an exile - the aim was reabsorption of the individual self-consciousness back into the divine consciousness. Man conceived himself as as 'a worm', with the merest glimmer or vestige of autonomy - and that autonomy essentially wicked.
By stages, over many centuries, the separation of self-consciousness and awareness of the self as unique increased until it became almost (but never fully) complete; so that now and for many generations Man regards himself no longer as a worm, but as the only god - which either leads to absolute (but brittle) pride at his self-creation of his own reality out of nothing; or (and eventually) to despair at his belief that therefore reality depends on his own continuous creation and is therefore feeble and temporary and doomed to end with death - Man regarding himself as something even-less-than a worm.
At this stage theosis has stopped, is no longer a purpose, life has no meaning outside of the contigent and ephemeral and private subjective consciousness.
This is the adolescence of Man.
Final Participation is the renewal of a new kind of theosis in which God and the Self are both regarded as real (eternally real) - and there are many selves, each on the path towards divinity. So the aim is not immersive participation in divine energies; it is not reabsorption into the divine; but the aim of Final Participation is instead to participate in the process of ever more, and ever more loving and creative, relationships between the many eternal selves of Men on the one hand and God (in divine multiplicity) on the other hand.
Final Participation is Final because the system is no longer closed (as it was in Original Participation) but open-ended and capable of eternal expansion, as we as individuals each and collectively grow towards a divinity of the same kind and level as God - but an unique, and continually added-to divinity; and with many others (being added-to) all around us, in relationships with us, who are doing the same.
To move towards Final Participation we need to consider the nature of our relationship with the divine - and that we are to understand ourselves as immature and very-partial divinities - but that God has a loving and paternal relationship with us; so we need have nothing to fear from him and an attitude of trust and confidence in him as he will always want the best for us and work for that end.
For Final Participation, therefore, we need to see God as a person and a personal friend; and not somebody or some-thing vast and mysterious to be awed by and needing to be appeased, not somebody to be pleaded-with, nor an alien and incomprehensible being to be worshipped - and not an abstract infinite perfection which we seek to 'lose ourselves' into. At least, such attitudes cannot be foremost and regulative of our relation to God - but only background, exceptional and temporary.
Of Course, God condescends greatly to meet us at our level, and for that we should be grateful; but having said that we just need to put aside that fact and get on with the relationship at our own childish or adolescent level (just as a child knows that the adult is condescending to play, but the play cannot be play unless that condescension is 'forgotten' while the play is in progress). Respectful friendliness, trust, confidence - and an 'equality' which (like the child's in play with parent, as he grows) is not less real for continually being superceded by higher levels of maturing and diminishing magnitudes of difference.
Barfield - following Coleridge - saw reality in terms of distinguishable, dynamic but not separable polarities. The Polarity of Final Participation may be between God as an eternal and fully-divine person; and each of ourselves as eternal and partially-divine persons. The poles never to be united, but always bound-together in dynamic process, energized by that thing we could call Love - so long as we are clear that Love contains many positive aspects such as creativity, intelligence, power...
In sum - the movement from Original to Final Participation (leaving-out the long transitional state that occupies recorded history, and in which we still seem to be 'stuck') is therefore centred on the work of Christ; understood as enabling the change from theosis as loss of the self and reabsorption back-into the divine - to theosis as a stronger and maturing self-awarness and consciousness; closer and closer towards the adulthood of a full friend-like relationship between the personal loving God and his growing-up child.
It is the lived experience of this theosis which is Final Participation.
Note added: Having posted the above I almost immediately came upon an explicit confirmation of my interpretation on page 154 of What Coleridge Thought by Owen Barfield:
...The polarity God [polar-with symbol] Man is the basis of all polarity, in nature and elsewhere.
This leads to my final summary:
Original Participation = Divine Unity
Consciousness Soul = Human Separateness
Final Participation = Divine-Human Polarity