A much quoted Tolkien letter described his goal in writing his large and uncompleted 'Silmarillion' legendarium: I had a mind to make a body of more or less connected legend, ranging from the large and cosmogonic, to the level of romantic fairy-story-the larger founded on the lesser in contact with the earth, the lesser drawing splendour from the vast backcloths – which I could dedicate simply to: to England; to my country.
The need for Tolkien to embark on this vast project is described in another part of this letter: I was from early days grieved by the poverty of my own beloved country: it had no stories of its own (bound up with its tongue and soil), not of the quality I sought and found (as an ingredient) in legends of other lands.
What Tolkien meant specifically was the pre-Christian Anglo Saxon folklore, myths, legends and religion. But we could extend our regret to the whole vast era of pre-Roman (and indeed Roman) Britain - going back through the druids of the Iron Age 'Celts' through the transitional bronze age; to the builders of the megalithic monuments and that vast and elaborate sacred landscape of linked and orientated standing stones (and wooden structures), earthworks and enclosures, causeways, hills, ridgeways and tombs that occupies so much of southern and central England (as well as being found in many other parts of the British Isles).
So much has been lost! And we may feel, like Tolkien, that what was lost is important - indeed, that we really need to know what was lost, and because we don't know it we are impaired, stunted, unable to proceed to where we ought to gone.
The problem is most acute for England specifically; because of the Norman conquest, which eradicated, all-but obliterated, Anglo Saxon high culture. And was compounded by Henry VIII's depredations and the activities of Puritan iconoclasts.
The scholarly approach has recovered much of value - both from archaeology, from careful analyses and comparison of documents, place names, dialect words and other fragments (including the deployment of philology - Tolkien's discipline). Yet there is so much missed, which seems that it is probably important.
Why important? The unspoken aim of the yearning that all this triggers is perhaps that we feel a need to know in order that we might pick-up the broken threads, and resume a development that was prematurely broken-off... to our loss. Because it was broken-off, because the knowledge was lost, it feels like the deficit is incurable...
Well, leaving aside - or building-around - the world of scholarship and rational extrapolations of material 'evidence'; I think we can be confident that - in one way or another, and insofar as it is important and necessary - recovery of lost knowledge can be done. Indeed, not just 'can' but it will be done, if we sincerely seek it. (Sincerely = for the sake of destiny and not for our benefit.)
So we need not worry, but we should pay attention; pay attention to our own, personal, intuitive thinking which (by some version of synchronicity) will lead to to exactly what we need.
But why should we be confident of this? Very simply, because as Christians we can have faith in our loving God, the creator, our Father; who will arrange matters such that we (his children) are not thwarted by lack of knowledge which may be lost to us, but is known to him.
All God has to do is communicate it to us; all we have to do is seek and understand it.
Note: It may be asked why we need to seek for this knowledge, if it is important? Why don't we know it already, or why doesn't God ensure that everybody is told it explicitly? The answer is profound, and to do with the essence and purpose of this world; which is for our experience and learning. Striving, seeking, overcoming... these are vital aspects of our learning. Therefore the typical pattern is that God wants us to do as-much-as-possible for ourselves - and 'help' is only provided when absolutely required.