Eru Ilúvatar

Eru Ilúvatar is a fictional character in J.R.R. Tolkien's Middle-earth legendarium. He is introduced in The Silmarillion as the supreme being of the universe, creator of all existence. In Tolkien's invented Elvish language Quenya, Eru means "The One", or "He that is Alone" and Ilúvatar signifies "Father of All". The names appear in Tolkien's work both in isolation and paired (Eru Ilúvatar).

Eru as Creator
Eru was the supreme being, God. Eru was transcendent, and completely outside of and beyond the world. He first created a group of angelic beings, called in Elvish the Ainur, and these holy spirits were co-actors in the creation of the universe through a holy music and chanting called the "Music of the Ainur", or Ainulindalë in Elvish.

Eru alone could create independent life or reality by giving it the Flame Imperishable. All beings not created directly by Eru, (e.g., Dwarves, Ents, Eagles), still needed to be accepted by Eru to become more than mere puppets of their creator. The evil Melkor, who had originally been Eru's most powerful servant, desired the Flame Imperishable and long sought for it in vain, but he could only twist that which had already been given life.

Eru created alone the Elves and Men. This is why in The Silmarillion both races are called the Children of Ilúvatar. The race of the Dwarves was created by Aulë, and givensapience by Eru. Animals and plants were fashioned by Yavanna during the Music of the Ainur after the themes set out by Eru. The Eagles of Manwë were created from the thought of Manwë and Yavanna. Yavanna also created the Ents, who were given sapience by Eru. Melkor instilled some semblance of free will into his mockeries of Eru Ilúvatar's creations (Orcs and Trolls).

Eru's direct interventions
In the First Age, Eru created and awoke Elves as well as Men. In the Second Age, Eru buried King Ar-Pharazôn and his Army when they landed at Aman in S.A. 3319. He caused the Earth to take a round shape, drowned Númenor, and caused the Undying Lands to be taken "outside the spheres of the earth". When Gandalf died in the fight with the Balrog in The Fellowship of the Ring, it was beyond the power of the Valar to resurrect him; Eru himself intervened to send Gandalf back.

Discussing Frodo Baggins' failure to destroy the Ring in The Return of the King, Tolkien indicates in Letter 192 that "the One" does intervene actively in the world, pointing to Gandalf's remark to Frodo that "Bilbo was meant to find the Ring, and not by its maker", and to the eventual destruction of the Ring even though Frodo himself failed to complete the task.

Tolkien on Eru
Peter Hastings, manager of the Newman Bookshop (a Catholic bookshop in Oxford), had written to Tolkien objecting to his writing of the reincarnation of Elves, saying:

In a 1954 draft of a reply to Hastings, Tolkien, also a devout Roman Catholic, defended his creative ideas as an exploration of the infinite "potential variety" of God: that it need not conform to the reality of our world so long as it does not misrepresent the essential nature of the divine:

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 simply as "He is", saying that this seemed to be a reference to the Biblical quotation "I Am that I Am", implying that Bombadil was God. Tolkien denied this:

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.

Inspiration and development
In earlier versions of the legendarium, the name Ilúvatar meant "Father for Always" (in The Book of Lost Tales, published as the first two volumes of The History of Middle-earth), then "Sky-father", but these etymologies were 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", published in Morgoth's Ring, the tenth volume of The History of Middle-earth.