[obloquy]

Could it be that Eru incarnate would be vulnerable to physical injury and death in the same way as was Christ? Would he have to obey the same physical laws as all inhabitants of Arda if he came into his own creation? The Athrabeth points out that Eru would need to be at once inside and outside Arda, thus being divided and presumably reduced in potency in the incarnate form; so it bears consideration that perhaps Eru incarnate could be stabbed in the back and physically killed.

I think Iluvatar would enjoy de facto invincibility regardless of whether his "incarnation" entailed diminution. Iluvatar's power, if it was not unlimited, was at least immeasurably greater than that of all other beings. His will (or power on the spiritual plane) could not be subverted or overcome, and therefore his power (on the physical plane) must be absolute. If anyone could have resisted his will it would have been Melkor in his primeval state, and yet Iluvatar claims that Melkor's resistance was vanity and that His own will would prevail despite him. Even when Melkor creates dissent, he is incapable of challenging Iluvatar for primacy and must have been aware of this: certainly Iluvatar did not create a being greater than himself.

The above, in order to respond directly to your hypothetical question, takes for granted Iluvatar’s incarnation. However, if Iluvatar needed to enter his creation, there would be little reason for him to become fully incarnate. A discarnate spirit need only assume physical "clothing" in order to interact with the physical. This avatar might be vulnerable to physical attack, but even if its matter was rendered useless the resident force would remain unharmed, and nothing would prevent Iluvatar from retaking shape. And yet we are still taking certain things for granted: we are supposing that Iluvatar would enter into the world in the same manner that his discarnate creations did, which is quite an assumption.

That said, I even disagree with the idea that Iluvatar would have to enter Arda to repair Melkor’s mar. Why might he? It’s a tempting idea for obvious reasons, but unlike Judeo-Christian theology, Tolkien’s mythology does not create this necessity. In the Bible, Christ (believed to be God by most, including Tolkien) was required to enter the world in order to provide a ransom for and redeem mankind. There is no such demand for perfect justice in the Legendarium. Finrod and Andreth give us Athrabeth at a time when Melkor runs rampant, uncontested; in their eyes Melkor’s defeat would require the physical presence of Iluvatar himself, the only being mightier than Melkor. That is not how things pan out, however. Melkor diminishes himself as the Morgoth and is eventually overpowered and imprisoned. According to Myths Transformed Melkor’s “person” (my term) is eventually executed, leaving only his disseminated power and his “taint” that is woven into the fabric of the physical world. We don’t know exactly how this might have been ultimately corrected, but I can see no benefit to Iluvatar’s physical presence within Arda. In fact, I believe the subversion of the wills of all other metaphysical powers (those Valar weaker than Melkor—in other words, all of them) in the primeval Music and the subsequent corruption of all physical creation can only be corrected by a power untouched by this physical aspect, and ever unaltered by this metaphysical influence. I think, therefore, that Arda can only be unmarred in the manner originally conceived by Iluvatar, which did not actually entail his personally overthrowing Melkor as Andreth speculated would be necessary; rather, Melkor essentially overthrew himself: gradually he squandered his power on evil, and eventually the strength of Good was able to overtake him. Iluvatar’s grand design for the complete mending of Arda might have been similarly gradual, subtle, and poetic.