From The History of Middle Earth Volume 10 - edited by Christopher Tolkien - excerpted from pages 330-333.
With regard to King Finrod, it must be understood that he starts with certain basic beliefs, which he would have said were derived from one or more of these sources: his created nature; angelic instruction; thought; and experience.
1. There exists Eru (The One); that is, the One God Creator, who made (or more strictly designed) the World, but is not Himself the World. This world, or Universe, he calls Eä, an Elvish word that means 'It is', or 'Let It Be'.
2. There are on Earth 'incarnate' creatures, Elves and Men: these are made of a union of hröa and fëa (roughly but not exactly equivalent to 'body' and 'soul'). This, he would say, was a known fact concerning Elvish nature, and could therefore be deduced for human nature from the close kinship of Elves and Men.
3. Hröa and fëa he would say are wholly distinct in kind, and not on the 'same plane of derivation from Eru', but were designed each for the other, to abide in perpetual harmony. The fëa is indestructible, a unique identity which cannot be disintegrated or absorbed into any other identity. The hröa, however, can be destroyed and dissolved: that is a fact of experience. (In such a case he would describe the fëa as 'exiled' or 'houseless'.)
4. The separation of fëa and hröa is 'unnatural', and proceeds not from the original design, but from the 'Marring of Arda', which is due to the operations of Melkor.
5. Elvish 'immortality' is bounded within a part of Time (which he would call the History of Arda [Arda is roughly the earth and solar system]), and is therefore strictly to be called rather 'serial longevity', the utmost limit of which is the length of the existence of Arda. A corollary of this is that the Elvish fëa is also limited to the Time of Arda, or at least held within it and unable to leave it, while it lasts.
6. From this it would follow in thought, if it were not a fact of Elvish experience, that a 'houseless' Elvish fëa must have the power or opportunity to return to incarnate life, if it has the desire or will to do so. (...)
7. Since Men die, without accident, and whether they will to do so or not, their fëar must have a different relation to Time. The Elves believed, though they had no certain information, that the fëar of Men, if disembodied, left Time (sooner or later), and never returned.
[Finrod] uncovers a concomitant tradition that the change in the condition of Men from their original design was due to a primeval disaster, about which human lore is unclear, or Andreth is at least unwilling to say much. He remains, nonetheless, in the opinion that the condition of Men before the disaster (or as we might say, of unfallen Man) cannot have been the same as that of the Elves.
That is, their 'immortality' cannot have been the longevity within Arda of the Elves; otherwise they would have been simply Elves, and their separate introduction later into the Drama by Eru would have no function.
He thinks that the notion of Men that, unchanged, they would not have died (in the sense of leaving Arda) is due to human misrepresentation of their own tradition, and possibly to envious comparison of themselves to the Elves.
For one thing, he does not think this fits, as we might say, 'the observable peculiarities of human psychology', as compared with Elvish feelings towards the visible world.
[Tolkien refers here to Finrod's observations that (in these respects, being different from elves) Men seem to feel they are visitors to the earth (Arda), not 'at home', in exile, perpetually dissatisfied, rapidly wearying of things, seeking of novelty, seeking of a satisfaction on earth which they never can achieve... From this he infers that men were not made for this world only.]
[Finrod] therefore guesses that it is the fear of death that is the result of the disaster. It is feared because it now is combined with severance of hroa and fea.
But the fear of Men must have been designed to leave Arda willingly or indeed by desire - maybe after a longer time than the present average human life, but still in a time very short compared with Elvish lives.
Then basing his argument on the axiom that severance of hroa and fea is unnatural and contrary to design, he comes (or if you like jumps) to the conclusion that the fea of unfallen Man would have taken with it its hroa into the new mode of existence (free from Time).
In other words, that 'assumption' was the natural end of each human life, though as far as we know it has been the end of the only 'unfallen' member of Mankind.
[Tolkien refers here to Mary, the Mother of Jesus; and the ancient Catholic tradition that she died willingly and was bodily assumed directly to Heaven. However, the Eastern Orthodox Catholic tradition would not agree with Tolkien's Roman Catholic belief that Mary was 'unfallen'.]
I regard JRR Tolkien as one of the wisest and most profound of men, and further I take the above discussion seriously as an attempt - within the subcreation of his Legendarium - to grapple with ultimate matters.
Furthermore, I find his reasoning compelling.
Note what he says about the necessary assumptions. In the case of the Elven King Finrod, these assumptions were based on his created nature; angelic instruction; thought; and experience.
In the case of Men (who have not lived among the 'angels' (Valar) as had elven King Finrod; the assumptions would be based on created nature, thought, experience - and any traditions concerning divine 'revelation'.
His conclusion is that the Fall (conceived as a turning away from God, and a worship of the Satanic figure of Melkor/ Morgoth - which is a turning away from love to power) led to fear of death, as a severance of (immortal) soul and (mortal) body which is unnatural and horrible.
Eru's original plan was that this would not have happened, but that Men on willingly accepting death at the end of their time on earth would go (body and soul) to another world (i.e. Heaven) which was out of Time.
Following the fall, and a time of fallen-ness where Men's souls were indeed severed from bodies at death, an alternative plan was devised by Eru whereby he himself would become a Man, and thereby (mystically) enable souls which had left the world without their bodies to be reunited with their proper bodies, using (roughly speaking) the 'memory' of the body which was retained by the soul.
This was (Tolkien explains elsewhere) the mechanism for elven reincarnation - a new body was 'regenerated' from the memory of the soul.
But the souls of Men were not like this (the special elven gift was memory), nor was reincarnation the destiny of Men.
After all, the souls of dead Men had left Arda (whereas elven souls remained in Arda), and were in a domain out of Time.
Only intervention by Eru could heal this situation, and any healing must allow for the free will of Men (which was part of the essence of Men and the 'reason' or purpose of their creation).
Against this was not just the free will of Men to reject any or all of the assumptions or to prefer power to love; but there was also the fact of the presence of evil in the fabric of the world (the tainting of the created world by Morgoth); the purposive evil of Morgoth himself, his allies (Sauron) and his corrupted servants - Balrogs, Dragons, Orcs; and the opposition of free Men who (each, by choice or assent) took Morgoth as their God.
This is Tolkien's indirect description of the Fall and Resurrection; and his explanation of the need for Resurrection.
Inter alia, Tolkien's description of Finrod's assumptions is also a description of Faith (belief in the reality of Erus and his nature), Hope (called Estel - by which knowledge of Eru implies goodness of divine purpose) - and the distinctive Christian virtue of Charity (Love, Agape) is implied by the contrast with Pride and Power-seeking which are distinctive sins of the two Falls of Men - the primary fall of the worship of Morgoth in the unrecorded history, and the secondary historical fall of Numenor into pride and power - finally capped by the Numenorean King again reinstating the worship of Morgoth - supervised by Morgoth's priest Sauron.