[Angry Hill Troll]

The Incarnation of God is an infinitely greater thing than anything I would dare to write.

Tolkien did, however, refer rather explicitly to the possibility of this happening at some point in Arda's future, in the Athrabeth. The topic is discussed in this thread. Finrod points out that it is unlikely that Eru could ever be completely contained in a physical form, which would necessarily be finite.

..it seems to me, even if He in Himself were to enter in, He must still also remain as He is: the Author without.

Note also that in the oral tradition of the Edain referred to by Andreth "the Tale of Adanel" the Voice of Eru spoke to men before they met any of the elves or Ainur. So it may not be strictly true that the only knowledge of Eru which men possessed came from the elves.

It isn't clear just what the phrase "his [Eru's] face was terrible to behold" means, since Eä had not yet been created, and it doesn't appear likely that the Ainur would have had physical form either, during the music. A short time later the text says "..he showed to them a vision, giving to them sight where before was only hearing..." Does this mean that before this point the Ainur did not possess sight, or only that previously they were not able to see what the Music entailed? In any event, I think that "face" doesn't really imply a physical form, at least not in the sense that they exist within Arda.