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- "But Rúmil said: 'Ilúvatar was the first beginning, and beyond that no wisdom of the Valar or of Eldar or of Men can go.' 'Who was Ilúvatar?' asked Eriol. 'Was he of the Gods?' 'Nay,' said Rúmil, 'that he was not, for he made them. Ilúvatar is the Lord for Always who dwells beyond the world; who made it and is not of it nor in it, but loves it.' "
- ― The Book of Lost Tales Part One, "The Music of the Ainur"
He was considered the father of the Ainur, thus in lineage charts Ainur are shown as Children of Ilúvatar. However, not all of the Ainur were considered to be siblings. For instance, Manwë, Varda, and Melkor's father was Ilúvatar, and Melkor and Manwë were considered brothers; Varda was not considered their sister.[source?]
Eru created the Ainur before anything else. To them he presented his thought in the form of music, and listened as the Ainur picked up his themes and elaborated on them, slowly learning to sing in harmony with each other. Eventually he showed them his greatest theme, and bade them sing it in harmony and develop it with newly granted powers. Out of this great music, Eä, the universe, was created, and within it the Valar created Arda. 
Elves and Men were created by Eru directly, without delegation to the Ainur, and they are therefore called "Children of Ilúvatar" (Eruhini). The Dwarves were "adopted" by Eru in the sense that they were created by Aulë but given sapience by Eru. Animals and plants were probably fashioned by Ainur after themes set out by Eru in the Music of the Ainur, although this is questionable in cases where animals exhibit sapience, as in the case of Huan, or the Eagles.
The activities of Eru on the life of Arda or Eä is not clear. Manwë was the vicegerent of Eru on Arda. The Changing of the World was made by Eru, something that the Valar themselves could not have done. According to Athrabeth Finrod ah Andreth Eru would someday enter Eä to save his Children. It is said that after the end of days, Eru will unite the Ainur and the Children of Ilúvatar to create a music even greater than the one of creation.
It is to be noted that in earlier works of the legendarium the name Ilúvatar meant "Sky-father" since the element il- refers also to the sky (cf. Ilmen), but this etymology was dropped in favour of the newer meaning in later revisions. Ilúvatar was also the only name of God used in earlier versions — the name Eru first appeared in the Annals of Aman.
Tolkien understood Eru not as a "fictional deity" but as a name in a fictional language for the actual monotheistic God, although in a mythological or fictional context. In a draft of a letter of 1954 to Peter Hastings, manager of the Newman Bookshop (a Catholic bookshop in Oxford), Tolkien defended non-orthodox aspects as rightly within the scope of his mythology, 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"
- ― Peter Hastings
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!"
- ― J.R.R. Tolkien, Letter 153
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.
- ↑ 1.0 1.1 J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Quenta Silmarillion: Of the Beginning of Days"
- ↑ J.R.R. Tolkien, "Qenya Noun Structure", in Parma Eldalamberon XXI (edited by Christopher Gilson, Patrick H. Wynne and Arden R. Smith), p. 83
- ↑ J.R.R. Tolkien, Christopher Tolkien (ed.), Morgoth's Ring