Tolkien understood Eru ["Eru Ilúvatar" is the name of God, the single omnipotent creator in Tolkien's legendarium] not as a fictional deity but as a name in a fictional language for the actual monotheistic God of Christian faith, although in a fictional context. A clear explanation of this appears in a draft of a letter that Tolkien wrote in 1954 to Peter Hastings, manager of the Newman Bookshop (a Catholic bookshop in Oxford). In the letter, Tolkien, a devout Catholic, defended the non-orthodox portrayal of God (Eru) in his writing as rightly within the scope of his legendarium, as an exploration of the infinite "potential variety" of God.
Regarding the possibility of reincarnation of Elves, Hastings had written:
"God has not used that device in any of the creations of which we have knowledge, and it seems to me to be stepping beyond the position of a sub-creator to produce it as an actual working thing, because a sub-creator, when dealing with the relations between creator and created, should use those channels which he knows the creator to have used already."
Tolkien's reply contains an explanation of his view of the relation of (divine) Creation to (human) sub-creation:
"We differ entirely about the nature of the relation of sub-creation to Creation. I should have said that liberation "from the channels the creator is known to have used already" is the fundamental function of "sub-creation", a tribute to the infinity of His potential variety [...] I am not a metaphysician; but I should have thought it a curious metaphysic — there is not one but many, indeed potentially innumerable ones — that declared the channels known (in such a finite corner as we have any inkling of) to have been used, are the only possible ones, or efficacious, or possibly acceptable to and by Him!"
Hastings had also criticised the description of Tom Bombadil by Goldberry: "He is", saying that this seemed to imply that Bombadil was God.
Tolkien replied to this:
"As for Tom Bombadil, I really do think you are being too serious, besides missing the point. [...] You rather remind me of a Protestant relation who to me objected to the (modern) Catholic habit of calling priests Father, because the name father belonged only to the First Person."