If Eru is the creator of all that is, does this make him the most powerful force in the Tolkien universe? Has he created beings, such as Sauron, which have more power than him?
Eru is far and away the most powerful entity in the legendarium
Why? Well (All quotes are from the Silmarillion)...
Eru created the Ainur
There was Eru, the One, who in Arda is called Ilúvatar; and he made first the Ainur, the Holy Ones, that were the offspring of his thought, and they were with him before aught else was made.
The plan for the entire world was known only to Eru
And he spoke to them, propounding to them themes of music; and they sang before him, and he was glad. But for a long while they sang only each alone, or but few together, while the rest hearkened; for each comprehended only that part of the mind of Ilúvatar from which he came, and in the understanding of their brethren they grew understanding, and increased in unison and harmony.
Morgoth, greatest of the Valar, tried to corrupt Eru's plans, and failed
The other had now achieved a unity of its own; but it was loud, and vain, and endlessly repeated; and it had little harmony, but rather a clamorous unison as of many trumpets braying upon a few notes. And it essayed to drown the other music by the violence of its voice, but it seemed that its most triumphant notes were taken by the other and woven into its own solemn pattern.
And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined.'
Only Eru could bring forth being from nothingness
To Melkor among the Ainur had been given the greatest gifts of power and knowledge, and he had a share in all the gifts of his brethren. He had gone often alone into the void places seeking the Imperishable Flame; for desire grew hot within him to bring into Being things of his own, and it seemed to him that Ilúvatar took no thought for the Void, and he was impatient of its emptiness. Yet he found not the Fire, for it is with Ilúvatar. But being alone he had begun to conceive thoughts of his own unlike those of his brethren.
Only Eru could bestow autonomy and consciousness
And the voice of Ilúvatar said to him: 'Why hast thou done this? Why dost thou attempt a thing which thou knowest is beyond thy power and thy authority? For thou hast from me as a gift thy own being only, and no more; and therefore the creatures of thy hand and mind can live only by that being, moving when thou thinkest to move them, and if thy thought be elsewhere, standing idle. Is that thy desire?'
Was Sauron greater?
Think of The Lord of the Rings within the scope of Catholic theology. Not as allegory—Tolkien detested allegory—but as a simple statement of the beliefs of its author. Iluvatar is God, Morgoth is Lucifer. Sauron is a powerful archdemon subservient to Lucifer. The question almost answers itself, does it not?
In Tolkien terms, Sauron was of a lesser order of Ainur, the Maiar. Morgoth was a Vala, and even he did not possess the Secret Flame.
Perhaps we need more direct proof? Once upon a time, Sauron sought to turn the Númenóreans to the worship of Morgoth, the Great Enemy (hint: equate enemy to "adversary," then try translating to Hebrew):
Thereafter the fire and smoke went up without ceasing; for the power of Sauron daily increased, and in that temple, with spilling of blood and torment and great wickedness, men made sacrifice to Melkor that he should release them from Death. And most often from among the Faithful they chose their victims; yet never openly on the charge that they would not worship Melkor, the Giver of Freedom, rather was cause sought against them that they hated the King and were his rebels, or that they plotted against their kin, devising lies and poisons. These charges were for the most part false; yet those were bitter days, and hate brings forth hate.
Ilúvatar was not obviously present in the world, which made it easy for the worship of Melkor or Sauron to take hold. But authority over the world was certainly still his.
Then Manwë upon the Mountain called upon Ilúvatar, and for that time the Valar laid down their government of Arda. But Ilúvatar showed forth his power, and he changed the fashion of the world; and a great chasm opened in the sea between Númenor and the Deathless Lands, and the waters flowed down into it, and the noise and smoke of the cataracts went up to heaven, and the world was shaken. And all the fleets of the Númenóreans were drawn down into the abyss, and they were drowned and swallowed up for ever. But Ar-Pharazôn the King and the mortal warriors that had set foot upon the land of Aman were buried under falling hills: there it is said that they lie imprisoned in the Caves of the Forgotten, until the Last Battle and the Day of Doom.
But the land of Aman and Eressëa of the Eldar were taken away and removed beyond the reach of Men for ever. And Andor, the Land of Gift, Númenor of the Kings, Elenna of the Star of Eärendil, was utterly destroyed. For it was nigh to the east of the great rift, and its foundations were overturned, and it fell and went down into darkness, and is no more. And there is not now upon Earth any place abiding where the memory of a time without evil is preserved. For Ilúvatar cast back the Great Seas west of Middle-earth, and the Empty Lands east of it, and new lands and new seas were made; and the world was diminished, for Valinor and Eressëa were taken from it into the realm of hidden things.
In an hour unlooked for by Men this doom befell, on the nine and thirtieth day since the passing of the fleets. Then suddenly fire burst from the Meneltarma, and there came a mighty wind and a tumult of the earth, and the sky reeled, and the hills slid, and Númenor went down into the sea, with all its children and its wives and its maidens and its ladies proud; and all its gardens and its bells and its towers, its tombs and its riches, and its jewels and its webs and its things painted and carven, and its lore: they vanished for ever. And last of all the mounting wave, green and cold and plumed with foam, climbing over the land, took to its bosom Tar-Míriel the Queen, fairer than silver or ivory or pearls. Too late she strove to ascend the steep ways of the Meneltarma to the holy place; for the waters overtook her, and her cry was lost in the roaring of the wind.
Sauron himself was stripped of his physical form in this cataclysm, and fled to his later abode of Mordor. It is perhaps worth noting that after this, though he strove for domination of humans, elves, orcs, dwarves, and generally all and sundry, he seems to have put significantly less emphasis on being worshiped as a god.
Besides casting down Sauron without effort, what Eru did, as indicated in the passages, was to:
- Destroy Numenor
- Remove entire places from the face of Arda
- Reshape Arda into a round world
- Preserve Ar-Pharazôn from dying until the end of days.
Eru, like any sufficiently powerful creator deity, is omnipotent and omniscient relative to the setting, and Tolkien basically hinted that he is God, or he is God as far as the residents of Middle-Earth are concerned. Specifically:
Since Tolkien intended Middle Earth to be our earth but in a mythical time period (akin to Dreamtime, Tir Na nÓg, etc.) it follows that Eä is a fictionalized version of our Universe
Tolkien intended Eä to be our entire Universe as it was known to the more knowledgeable ancient astronomers, not just composed of the mythical solar system with its upper and lower "airs" as it was known to early humans (who believed that the sun had to battle the snake of the underworld every day as it passed beneath the earth)
However this is a myth of how Middle Earth works in-universe. Tolkien reconciled the two by saying Eru changed the shape of the world from flat to round -- part of passing from myth-time (when the Valar sang the world into being) to historical time.
But even in the Ainulindale (the Elvish myth of the creation of the world) Eä contains "vast spaces" and "wheeling fires" and ancient stars created by Elbereth in the springtime of the Ainur's power and other unknown worlds where Morgoth was free to work his will.
Therefore Eä was the entire Universe, not a mere pocket universe or world in a bottle as might be ruled by a lesser deity. And it was all dreamed up by Eru (see below)
The key thing to remember about Tolkien is that "magic" is tied to spiritual power and subject to entropy. Everything was exponentially more powerful the further back in time you go. So the Valar at the start of creation were unimaginably powerful -- capable of creating entire planets, of which they chose one, Arda, for habitation.
Therefore since Tolkien was Catholic, the God of Eä is the Creator God of an alternate version of our Universe in an alternate time (whichever God does not matter, Eru's attributes are universalist and hence unitarian) for the same reason Allah and Aslan = the Catholic God according to monotheistic logic.
The "power" of spiritual beings in Tolkien's legendarium is only potentialized as it is expended on matter (hinted at in the text, but confirmed per Morgoth's Ring, which actually gives a midichlorian-like explanation for the quantity of "Morgoth Element" in gold used to make the One Ring). This is not mathematic, but qualitative and relative to other spiritual beings -- the Vala Ulmo was "in tune" with water, Manwë was in tune with air, etc.
It is implied that Eru actually lowered himself to interfere at such a base level as to merely destroy a continent (Númenor), which the Valar were quite capable of doing, but felt constrained not to murder the Children of Eru Illuvatar.
In the early texts, Tolkien mentions powerful Ainur stayed behind with Eru in the Timeless Halls of the (otherwise formless) Void, and that these halls (where Gandalf went when he died) are literally Timeless. These other Ainur (including anomalies out of the Void, like Ungoliant) literally exist in a realm beyond space and time, and possibly help define it (Ungoliant emitted an unlight that was physically impossible, for instance) so their "power" cannot be measured.
The final relevant thing is in the Ainulindale, which explains Tolkien's theodicy. Eru not only created all these beings, and inspired them to sing all this into being (in thought alone),
He alone had (and still has, c.f Dwarves, Men and Elves) the power to give their ideas physical existence and consciousness in the case of creatures. This is the meaning of the term Secret Fire (or Flame Imperishable)
But wait there's more! Eru presupposed the very thoughts of the Ainur, including the Valar and Morgoth. Eru knew the secret innermost thoughts of the Valar who
createdbuilt the world once it was formed and gave them all their ideas. He knew Morgoth would rebel and in so doing "create things more wonderful" which the Valar had not dreamed in their original conception of a perfect world.
In keeping with Tolkien's Catholic philosophy, only humans (and possibly Elves) truly have free will (in the sense of being able to go beyond fate as Eru decreed it). The most powerful of the Valar, essentially polytheistic beings, are incapable of doing or thinking anything that Eru has not already forseen.
So "power levels" don't even enter into it. Eru created the very idea of power as understood on Middle Earth. Since Tolkien viewed himself as a Sub-creator, Eru is more powerful than the author himself!