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.
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.