The God of Abraham
The three widely practiced monotheistic religions - Judaism, Christianity, and Islam - all trace their spiritual lineage to Abraham, and worship the God that revealed Himself to him: the God that, when asked, first identified Himself to Moses as "the God of Abraham, Isaac and Jacob." Although Buddhists revere Buddha, he didn't claim to be divine and most Buddhists don't consider him to be. Hindus are polytheistic. Most animistic and other tribal religions also believe in more than one divine spirit, although there are so many of those cultures that it's impossible to make an all-inclusive statement about their beliefs. In "On Fairy-stories," Tolkien said that God is the God of Men - and of Elves. As said in the previous essay, Eru isn't an "invented" God, but the God of Abraham, Isaac and Jacob as understood by the people of Middle-earth, who didn't have the benefit of His revelation to Abraham.
I've seen attempts to puzzle out how this could happen and to decide if it's valid: Is Middle-earth's history set in a time before Abraham? Does it occur after Abraham - or perhaps during the same historical time - but the revelation hasn't spread that far geographically? But if either of those is true, how can the residents of Middle-earth be worshipping the God of Abraham at all? When it's Christians doing the puzzling, that sometimes leads to the question of whether Eru, of necessity, isn't a false god.
Even in the Bible, there are reports of people living earlier than Abraham who had contact with God: Noah, for example. We also need to keep in mind that Tolkien's entire cosmos is a secondary creation, giving us insight into our primary creation but having no reality within it - not even at a time in our primary creation's remote past. Middle-earth doesn't exist before, during, or after the time of Abraham, because it doesn't exist in his (our) reality at all. This will be talked about more in the section of essays on inspiration ('...by which I do not mean myself...'), and I'll use the following quote, there, too. It's from letter #153, written in response to a Catholic who said that the reincarnation of Elves was "heretical" - among other complaints:
You have at any rate paid me the compliment of taking me seriously; though I cannot avoid wondering whether it is not 'too seriously', or in the wrong directions. The tale is after all in the ultimate analysis a tale, a piece of literature, intended to have literary effect, and not real history. That the device adopted, that of giving its setting an historical air or feeling, and (an illusion of?) three dimensions, is successful, seems shown by the fact that several correspondents have treated it in the same way- according to their different points of interest or knowledge: i.e., as if it were a report of 'real' times and places, which my ignorance or carelessness had misrepresented in places or failed to describe properly in others... [After 9 pages of discussing all kinds of things this reader thought were "wrong," there's a note saying: 'Not sent. It seemed to be taking myself too importantly.']
So, is Eru the God of Abraham, Isaac and Jacob? Yes. Why? Because Tolkien said He is (unequivocally and repeatedly in his letters), and it's Tolkien's subcreation. Again, this will be discussed more later, but it seems like a good idea to remind ourselves toward the beginning of this exercise that it defeats the purpose if we take Tolkien too seriously "in the wrong directions." As is evident from the fact that I'm writing all of this, I believe Tolkien's writing - especially The Lord of the Rings - can say some important and serious things to us if we allow it to, but those things don't include the actual "historical air or feeling." Although that device can initially draw readers into the subcreation, in the same way that the surface adventure story draws some others, it's not the reason we're there. A true secondary creation (as opposed to a good setting for a science fiction or fantasy story) shows us how the author perceives our primary creation. But it does this through the story itself, not through a historical connection - although a feigned historical connection such as Middle-earth's can increase the setting's sense of reality, when presented as well as Tolkien presents it.
I was recently reading a collection of pieces of literary criticism on the poem Beowulf. To be honest, I was reading it (and the person who'd given me the book had done so) because the first piece in the collection was Tolkien's "The Monsters and the Critics," from 1936, of which I'd seen bits and pieces but had never read the entire thing. I was hoping that reading it would give me some insight into Tolkien's own fiction, but I wasn't expecting the "a-ha" experience it gave me. In discussing it, though, I'm going to start from another essay in the collection, "The Christian Language and Theme of Beowulf" by Thomas D. Hill, which was first published in 1994.* It cites Tolkien's 1936 piece as support for some of its comments. There's a concept on which the two basically agree, and Hill uses a label when discussing it that can save us some verbiage as we look at it here.
The concept is that the writer of Beowulf, who was Christian, was telling a story about his people's pre-Christian times but was telling it from a Christian perspective. In doing so, he didn't portray the earlier people as Christian, or as possessing any revealed knowledge of God, but he did portray them as monotheists holding values compatible with Christianity. Hill labels this depiction "Noachite,"** which he defines as "...gentiles who share the religious heritage and knowledge of Noah and his sons without having access to the revealed knowledge of God which was granted to Abraham, Isaac, and Jacob..." Neither the Beowulf poet nor Hill is saying that those ancestors actually lived during the historical time of Noah and his sons but, as a Christian writing about pagan ancestors, that's the level of religious knowledge the poet shows them having. In the historical time and place the Beowulf poet was writing about, this would have been factually untrue; those ancestors would have been polytheists, and he couldn't rewrite actual history.
When Tolkien wrote "The Monsters and the Critics" not even The Hobbit had yet been published, but he had already been developing his earlier mythology for about 20 years as a private hobby. When I read his comments about the Beowulf poet telling a story from a pre-Christian time but through a Christian perspective, I felt as if I was reading about Tolkien's own mythology. It's impossible to know whether Beowulf affected how Tolkien saw Middle-earth or whether Middle-earth affected how Tolkien read Beowulf, but since Hill read Beowulf the same way Tolkien had, nearly 60 years later, I'd say the first interpretation has some validity to it. Interestingly, Hill has found this same kind of writing in the Icelandic sagas, one of Tolkien's favorite sources (for example, the names of all of the Dwarves in The Hobbit come from there). So, in portraying the inhabitants of Middle-earth as Noachites Tolkien wouldn't have been doing anything that he would have seen as a new invention; it's a very old kind of writing and one with which Tolkien would have been very familiar.
In trying to distill some of Tolkien's and Hill's relevant comments into a few words, I would say that one reason for this kind of writing is to show respect for the previous culture and for the values it held that were compatible with Christianity, while ignoring values - and forms of worship - that were incompatible.
In his mythology, Tolkien has one big advantage over the Beowulf poet in that he's not looking at a pagan past that's historically true, but is creating a history that's true only within his stories. The Beowulf poet could write about his ancestors as if they were monotheistic Noachites - but they weren't. Tolkien, on the other hand, could create characters and cultures that actually are monotheistic and Noachite because he's... well... creating them. And, to take things a step further, he's writing about these monotheistic pre-Christians from a Christian perspective. The first example that comes to mind is the pre-Christian Rohirrim going to battle because it's the right thing to do (based on their Noachite values), even though they believe the act is doomed to failure. But, the story itself tells us, doing the right thing is never doomed to ultimate failure, even though it may seem that way at the time; that's showing us the Christian perspective.
Except for one brief passage, which most of the essays I read attribute to someone other than the original poet, Beowulf makes no mention of pagan worship; it's one of those things the poet ignores. But, because his ancestors weren't Christian, the poet couldn't (or, at least, chose not to) show them worshipping as Christians. The result is an absence of any form of worship in the poem, even though there are instances where the characters do speak as if they believe in God - at a Noachite level. Sound familiar? Tolkien has another advantage over the Beowulf poet in that he can give us a reason that his characters don't worship openly: the Númenorean belief that, with the destruction of their place of worship in Númenor, they can no longer truly worship or pray to Eru but can only remember and reflect on Him.
Tolkien himself (in letter #165) calls Middle-earth a 'monotheistic world of "natural theology".'*** It's interesting that in this letter, written between the publications of The Two Towers and The Return of the King, he mentions that "The odd fact that there are no churches, temples, religious rites or ceremonies... will be sufficiently explained if... the Silmarillion and other legends of the First and Second Ages are published." I assume this refers to the destruction of Númenor and the effect this has on worship.
I didn't read these pieces on Beowulf until after I'd already written most of the essays in this collection, so this discussion is a late addition. But nothing I read contradicted anything I've already seen in Tolkien's writing, or specifically in LotR; it just gives a possible explanation for his choosing to write it the way he did - and one that makes a lot of sense to me, especially when put together with his statement that he omits showing any form of religion in Middle-earth specifically because he's a Christian. The fact that Tolkien was very knowledgeable about this type of writing from Beowulf and the Icelandic sagas adds to the likelihood of this explanation.
*Both of these pieces are contained in the "Criticism" section of Beowulf: A New Verse Translation by Seamus Heaney, 2000, W.W. Norton & Company.
**During the first days after I wrote this essay, I did some searching for information on the term "Noachite," and found that it has a couple of very specific meanings that I assume were not part of Hill's definition of the term. The first use of the word I discovered is as a title used by Freemasons, which we can pretty easily rule out in LotR! The second is closer to the relevant meaning here, although it's more specific, being used by Orthodox Jews to refer to the seven laws given by God to Noah and his sons after the Flood, as described in the Talmud. Depending on whose list you're reading, these laws are given various wordings, order, and emphases, but after condensing them as much as possible, they could be listed as follows: 1) Do not worship other Gods. 2) Do not blaspheme. 3) Do not kill. 4) Do not steal. 5) Avoid sexual misconduct. 6) Establish just courts. 7) Do not eat the limb of a living animal (this last one has interpretations ranging from the avoidance of animal cruelty to not eating meat that has the blood in it). There was a raging debate on Wikipedia for awhile - a couple of years ago, judging by the dates of the posts - on whether these Noachite laws are what's being referred to in the Acts of the Apostles as the rules to be given to Gentiles who are becoming Christians. An interesting idea, because the Jewish belief is that these laws were given to all people, and Gentiles who follow them will have a place in the world to come. The Wikipedia debate, BTW, didn't seem to resolve the issue.
***"Natural theology" has a very specific meaning in Catholic vocabulary, and Tolkien was a knowledgeable enough Catholic to realize that and to know exactly what he was saying here. The fact that he put the term in quotation marks supports that. Natural theology refers to what people without direct revelation from God can discern about God from His creation, and from what He has "written upon their hearts." It's often used in reference to the ethics and values such people live by, which is certainly an important aspect of it in LotR.