Being raised Protestant, one of the first things that I noticed about Ainulindalë as a creation myth is that in both Ainulindalë and Genesis, the world comes into being as the manifestation of the thoughts of an Original Divine Being. In Genesis, God’s cognitive process manifests itself in Word (which, according to the Gospel of John, is with God from the Beginning and was God), and through the Word, the things of this world were created. In Ainulindalë, on the other hand, the process is more convoluted. Eru’s thoughts themselves, not his words, are manifested in the Ainur, who are not synonymous to Eru in the fashion of the Christian Trinity.
Because of that, the existence of the Ainur brings up an interesting dilemma of free will. On one hand, they are progenies of Eru’s thoughts, each comprehending “only that part of the mind of Illúvatar from which he came” (The Silmarillion, 3), and therefore not beings that are ever completely independent of Eru nor ones that share in the Divine-ness of Eru in the way of the Trinity. In that sense, the Ainur can be said to be merely the instruments that brought about the things of the World through the music of creation (aside from Elves and Men, who were created by Illúvatar himself). However, after Eru reveals his grand theme to the Ainur, he declares to the Ainur:
"Of the theme that I have declared to you, I will now that ye make in harmony together a Great Music. And since I have kindled you with the Flame Imperishable, ye shall show forth your powers in adorning this theme, each with his own thoughts and devices, if he will. But I will sit and hearken, and be glad that through you great beauty has been waken into song.”
With that gesture, Eru seems to be acknowledging that the Ainur do possess some level of free will, or else he would have simply left off “if he will” and made the sentence a command. And with the additional fact that Eru himself is going to sit back, listen, and appreciate the beauty of the song that the Ainur make, it seems to suggests that more or less, the song of the Ainur can be thought of as much as a process of secondary creation as it was an expansion and lyrical manifestation of Eru’s great plan.
Also, taking into account Aulë’s creation of the dwarves, the problem at hand seems like it’s not one of the Ainur not really able to defy Eru’s intentions, but closer resemble the classic theological debate of free will vs. predestination—or rather, what happens when one’s thoughts take on an existence of their own, if the one in question is Illúvatar? In Chapter 2 of Quenta Silmarillion, of the beginning of the dwarves, it’s said that Aulë created them “because he was unwilling to await the fulfillment of the designs of Illúvatar” and the dwarves didn’t turn out identical to Elves or Men because “the forms of the Children who were to come were still unclear to his mind” so he filled in the blanks with his own designs (The Silmarillion, 40). In this story, Aulë doesn’t appear too different from the human protagonists of various scriptural stories who act out of their own accord as a result of impatience. However, Illúvatar knew what was done “in the very hour that Aulë’s work was complete” (40) and was pleased, which begs the question of how much of it was really Aulë not acting according to Illúvatar’s plan if Illúvatar, the originator of the piece of his thought that is Aulë, knew and “was pleased” despite the show of admonition later. Also, from the quoted passages earlier, it's easy to see that the only reason Aulë didn’t try to make cave-dwelling elves instead is because he didn’t know what the Children of Illúvatar are going to look like yet, it really begs the question of whether we even should be talking about free will, or is it a more complicated matter of instruments of creation “malfunctioning,” for lack of a better word, because said instruments are made from segments of Eru’s thoughts and are taking on a separate existence? And for that, I have no answers.
However, free will or no, there seems to be no question that while the Ainur and the Word might appear similar at first glance on the ground that they are both the manifestation of a Divine Being’s cognition, they are fundamentally different in status and function. The most decisive difference, I think, is in where the Father-Son/Creator-Children relationship is placed. As we all know, in the Bible, the Word, as something born from the thoughts of God, is sometimes referred to as the Son of God. In terms of human beings’ original sin and salvation through the sacrifice of Jesus, that distinction plays a crucial role because, using Thomas Aquinas’s theological interpretations, while human beings descended from Adam were created in the image of God, only the Word/Jesus is truly like God, and hence divine. The relationship between God the Father and God the Son in Christianity, on many levels, represents the model of perfection that the imperfect, sinful human beings can only achieve through Christ and his salvation.
With that in mind, the distinction of Elves and Men as the Children of Illúvatar seems a crucial one because in this configuration, while Illúvatar does not necessarily play a particularly active part in Tolkien’s legendarium, the basic undertone of the Creator-Creation relationship is nonetheless a more accessible one because in this creation myth, there isn’t a divine mediator who’s required for the Divine Being’s creations to attain the direct kinship between creator and creation. Elves, after all, can build a ship a sail to Valinor. The chief point of tension—and source of evil, if you will—under this configuration is therefore free will and desire, and not some all-encompassing original sin (which is interesting to think about in connection to Tolkien’s own Catholic faith, but that’s a topic for another day).