The recent collection of Tolkien's Numenor material into a single volume The Fall of Numenor (edited by Brian Sibley, 2022) has triggered considerable further thought concerning one of Tolkien's most profound mythic themes.
The significance of Numenor is something that I only gradually recognized, and which has increased over the years.
The reason is not hard to discover; because Numenor addresses Tolkien's core theme of "death"; because Numenor enables Tolkien to explore Man's response to death in a very pure situation of this-worldly bliss: an earthly paradise.
In Numenor, Men are given an ideal life in material terms: the "land of gift" bestows strength and stature, immunity from illness and the decline of age, greater intelligence and skill; and the best possible land and climate for humans to thrive.
The point is that in the rest of the world outside Numenor (as in our own world) there are always material 'reasons' to explain the insufficiency of life: things like sickness, violence, famine, old age etc. Men can therefore assume that "if only" the material conditions of life could be sorted-out - then Life would become completely satisfactory.
But Numenor is a world in which the material conditions have already been sorted-out; and yet Life is still insufficient!
In other words, in Numenor we are able to observe Men in a situation where all the solvable problems of life have been solved; and what remains are intrinsic features of the Human Condition.
We are thus invited to reflect upon: whether or not the situation of ideal Men in an ideal world is sufficient to satisfy our soul's need?
And Tolkien's answer is: No.
That is Tolkien's answer and I agree, as have many Men back (at least) to the times of the Ancient Greek philosophers. Numenor is an illustration of the fact that this mortal life is insufficient - no matter how ideal its circumstances. Men are not ultimately satisfied by paradise.
My understanding is that Man's eventual and decisive dissatisfaction with the life of Numenor was not itself evil: it was, indeed, an inevitability; and the fact that the Valar (and the Eldar) did not anticipate this dissatisfaction, and could not understand it once it had become evident - was evidence of the angelic powers' and the elves' limited sympathies when it came to Men: their limited understanding of The Nature of Reality.
Men's dissatisfaction with their life and this world is actually a consequence (albeit indirect and expressed by opposition) of their ultimate spiritual superiority to the Valar and the Eldar; and the reason why The One brought forth this second wave of 'humans beings': why the Followers (Men) were always intended and designed to replace the Firstborn (elves).
Death - in Tolkien's world - is called the Doom of Men; the word "Doom" covering both sides of the matter: that death was the ineradicable gift of Eru (God: the prime creator), and also that death was experienced as an inescapable and terrifying fate.
One lesson of Numenor is that the inescapable reality of death means that there can be no ultimately adequate life for Men - not even in Paradise. That recognition is wisdom.
But what then? If this mortal life is insufficient, if Paradise is not enough... What Then? How should Men understand their situation in the world; what should Men do?
Here is where the Men of Numenor went wrong - most obviously those who delusionally tried to attain eternal life by force of arms, but probably even those who were of "the faithful" - those who obeyed the Valar, and respected (and, it seems envied) the Eldar. Because Tolkien implies that "the faithful" wished in their hearts to be as the Eldar were, "immortal", but correctly recognized this was not possible.
This desire for elvish longevity made the faithful, and their Middle Earth descendants in Arnor and Gondor, a sad people, prone to childlessness and an excessive (also counter-productive) concentration on health and longevity.
Clearly it was a deep sin for the Men of Numenor to worship Morgoth and to assail the Valar. But; the Big Question is: what should the Men of Numenor have done instead?
Because, on the face of it: Men seem to be in a no win situation.
Men know that they are "doomed" to die, and know that their discarnate souls will leave "the circles of the world"; but Men have no idea (and have never been told by the Valar) what then (if anything) happens to their discarnate souls.
Is the future of all Men utter annihilation - or... something else?
This is indirectly addressed by Tolkien in his "Marring of Men" narrative - in which he has his protagonists allude indirectly to what we are intended to infer is the incarnation of Jesus.
Jesus is therefore put forward as being the - as yet unknown and apparently unknowable except by inference - answer to the problem of mortal Men and a world that has been "marred" ineradicably by Morgoth.
Yet it is important to realize that the answer "Jesus" - the resurrection and eternal life he brings that is only accessible via death - is, in Tolkien's universe, both unknown and unknowable to the Numenoreans. The Numenoreans have no foresight of Jesus, and neither do the Valar or the Eldar (who might, in principle, have informed Men).
It is assumed that only Eru foreknew the coming of Jesus, and Eru (apparently) did not tell anyone.
Men in Numenor are (in effect) asked to accept the insufficiency of mortal life on earth; and to hope without reason - to hope, based purely upon faith in the goodness of Eru.
This was the challenge of the Men of Numenor; and clearly they failed to respond rightly to that challenge; and in failing, brought nearly-complete ruin upon themselves and their civilization.
We can ask, however, whether (in Tolkien's world) it was reasonable of Eru to expect otherwise; given that He had not provided any of his creatures with any clue as the to eventual advent of Jesus Christ?
Was it, therefore, reasonable of Eru to expect Men to live by faith and hope yet without knowledge or assurance?
I sense that Tolkien was troubled by this aspect of his world; and was at least sympathetic with the Men who failed this high and ascetic task. And that Tolkien wondered why God did not provide his Men of Middle Earth and Numenor with foreknowledge of Jesus - which was indeed the situation for all Men except a tiny minority of Jews, in the ages before Christ.