Owen Barfield claimed not to be very clear about the nature of that Final Participation which he saw as the destiny of human evolution of consciousness.
(Original Participation is that immersive and unselfconscious living in-the-world of hunter-gatherers and similar tribal peoples, and modern man is detached from the world, and even his own thoughts, in the Consciousness Soul).
But I think that sufficient clarity can be arrived at by considering the phases of human love through human life as an analogy (or rather, as more than an analogy - since the one is an example of the other).
Infant and young childhood love is an Original Participation; immersive and undifferentiated - the child is hardly aware of himself as distinct from his parents and family; his experience of the world is of a world alive and sentient because he is himself a part of everything he perceives. Love is everywhere (in an ideal childhood) like a sea in which everything is immersed, or like a gas in which each individual thing is merely a slight concentration of the element. We know everything, but we hardly know our-selves, and everything blurs into everything else.
With adolescence comes self-consciousness, which intensifies until the Self experiences itself as detached from the world. Indeed the Self experiences most mental activity as detached from itself - our Selves are cut-off from all experience, looking out upon the world, such that they world seems uncertain and unreal (how do we know that it is real), and indeed the Self is also experienced as perhaps unreal - since it changes, and can only rely upon itself for validation.
The point of Consciousness soul is 'freedom' in the sense of agency: to be free we need to be able to live from our-Selves (our true Selves). The consciousness soul is the condensation and concentration of the self, such that we can work from it. This is, indeed, a divine state of being - and that is what this phase is necessary to human spiritual progression.
To evolve towards a fully divine state of being, we must pass-through the Consciousness Soul. But it is meant to be a transitional phase, a short-lived phase - a phase we merely touch upon in passing...
This Consciousness Soul state of consciousness is also the state of modern culture. Modern culture is stuck, arrested, in stasis in the consciousness Soul - so intensely subjective that subjectivity itself is destroyed - experiencing the external world only via perceptions and instruments, and unsure about the reliability of these senses and mechanisms... THE problem being epistemological - how do I know if I really know? No answer forthcoming (because the assumption is of a gulf between the self and the world) and Modern Man is stuck in Consciousness Soul - held, trapped, imprisoned by the assumptions of modern culture.
Modern Man is stuck in Consciousness Soul - and that was not, and is not, meant to be - the CS is meant to be just a phase (albeit a necessary phase) on the way to Final Participation. Being stuck in the Consciousness Soul is a precise analogy to being stuck in adolescence, unable to grow-up, paralysed in that state of alienation, in which uncertainty erodes all potential solutions - and unable to move on.
Final Participation is like the ideal of spouse - indeed married love is a type of Final Participation.
In married love at its highest, each individual retains the full self-consciousness of the Consciousness Soul, but has a certainty of contact with another, distinct, self-conscious human being. There is no problem of communication, indeed there is a certainty of communication. Each party is not trapped in the nutshell of their own skulls but is able to share, participate in the life of the other, but with full self-integrity and self-awareness.
On the one hand individuality is assumed and valued, and is part of the essence of married love; on the other hand there is no problem of communication between these individuals - because the universal immersive state of Original Participation still is real and exists as much as it ever did. We are still part of each other and of everything - but with Final Participation our individuality, our Self, has been clarified, strengthened... has become a free, autonomous agent.
Final Participation is Original Participation plus the Consciousness Soul - it is the sea of infant family love, plus the individual island of adolescent self-awareness.
There is no desire among loving spouses either for a return to the detachment of the Consciousness Soul (the state before love) or to return to the immersive and undifferentiated love of childhood family. They have the best of both worlds. The differentness and individuality and self-awareness of the other is essential, and so is the given background of universal communication.
In conclusion, when married love works as it is meant to, then it is a species of Final Participation - which is to say a state beyond Consciousness Soul, a moving-forwards and not a regression (or atavism).
Culturally, human destiny is to have an analogous relationship to ideal married love, but inclusive of all reality - and that is what Barfield meant by Final Participation.