Tolkien's Understanding of Eru

[Neurion]

I've always felt that Eru Illuvatar in Tolkien's works is meant to be a more or less direct reference to God in the Judeo-Christian tradition, but I'm wondering exactly what Tolkien himself ever might have said regarding Eru's nature. The Wikipedia article for Eru Illuvatar suggests that Tolkien considered Eru to actually be God, though viewed from the perspective of a fictional culture far in the past, and has this nifty quote from The Letters of J.R.R. Tolkien -

"We differ entirely about the nature of the relation of sub-creation to Creation. I should have said that liberation "from the channels the creator is known to have used already" is the fundamental function of "sub-creation", a tribute to the infinity of His potential variety [...] I am not a metaphysician; but I should have thought it a curious metaphysic — there is not one but many, indeed potentially innumerable ones — that declared the channels known (in such a finite corner as we have any inkling of) to have been used, are the only possible ones, or efficacious, or possibly acceptable to and by Him!"

- but I wonder, did Tolkien ever write anything else of this nature?