My best guess as to the central theme and destination of The Notion Club Papers (NCPs) is that it was to be about the Club re-establishing a spiritual connection between Elves and modern Men - perhaps via the intermediate civilization of Numenor.
And I understand this to be a version of Romanticism - the Romantic Project, dating back to the middle 1700s, from which time a strand within Western civilization began to explore and advocate such things as are explored and advocated (implicitly) within the NCPs.
For example the interest in myth and 'the primitive'; 'folklore' and nation; the imagination and intuition as ways of knowing; poetry, drama, novels and music as instruments of self-discovery and development; dreams, visions, epiphanies and suchlike regarded as spiritually valuable.
At the time the Notion Club Papers was last worked-on by Tolkien (1946) the civilization of The West, in particular its representation at Oxford University, was very different from what it has since become. Despite several generations of secularization; there was still a Christian basis underpinning much of English life (including many laws and other regulations); and Christianity had significantly revived during the Second World War.
For instance; CS Lewis and Charles Williams were both at the cutting edge of an intellectual revival of the Church of England, and other important figures in this movement were TS Eliot and Dorothy L Sayers - who were both friends and admirers of Charles Williams. As a Roman Catholic; Tolkien could point to the tremendous influence of (not long dead) GK Chesterton and (until recently active) Hilaire Belloc; and such prestigious literary converts as Evelyn Waugh and Graham Greene.
It would therefore have seemed reasonable to assume that the future of Christianity in England was at least secure and perhaps optimistic. At any rate, Tolkien apparently did not feel a need to emphasize, defend, or much mention Christianity in the NCPs - my impression is that it is taken-for-granted that the Club members were Christians.
In sum, therefore, the NCPs could be understood as about transforming Christian culture, especially in its creative and scholarly aspects (because the NCP members are creative writers, scholars and researchers of various types) by its becoming more Romantic; in particular (but not exclusively) by means of the 'enchantment' of Faery.
What actually happened was that from 1946; both Anglican and Catholic Christianity in England (and, even more so, in Oxford) was just entering a rapid and progressive decline that rendered it quantitatively and qualitatively insignificant to public life and discourse before the millennium; and by now has led to the de facto assimilation of most Christians to the hegemonic (encompassing the entirety of the mainstream, of all 'parties) totalitarian-materialist-leftist ideology.
This makes a crucial difference to the likely effect of a restoration of Romanticism such as the NCPs envisaged.
We know, from the revival of secular and non-Christian (especially Buddhist) Romanticism in the West, from the middle 1950s into the 1970s, that Romanticism without Christianity has failed to have a significant impact on the increasing materialism and bureaucracy of The West.
This has also - mostly - been the fate of Tolkien's own colossal cultural impact.
On the one hand, The Lord of the Rings is perhaps the most widely read and beloved book of the past seventy-something years - on the other hand, most of its readers and scholars are every bit as materialist, leftist, totalitarian-affiliated and modernity-assimilated as those who hate Tolkien.
Without Christianity to work-upon, Tolkien's Romanticism - despite its apparently vast influence - has not been able to stop or reverse the adverse trends in The West; not culturally, nor (with few exceptions) personally.
This suggests that the premise of the NCPs - that an actual re-connection between 'ancient' and Elvish influences and the modern world - would be A Good Thing; is not actually true in a society such as ours, from which Christianity (and indeed all effectively-motivated religion and transformative spirituality) has been deleted.
So on the one hand, Christianity without Romanticism did-not survive... And perhaps (I would say) could-not survive.
On the other hand, Romanticism without Christianity also makes little discernible difference; and operates as no more than a therapeutic or hedonic lifestyle-option within secular totalitarianism.
In reading the Notion Club Papers, to understand its intended implication; I think we need either to project ourselves backwards into the time it was written and the assumptions of that era; or else imaginatively to inhabit an alternative present or potential future; in which we can discover in our-selves that Christianity is still a living option.