[Lalwendë]

Without a precise theological statement, one could view Iluvatar either way, I think.

How I see it is that if we can view him either way, then we can only go on what evidence we have, and that is that Eru is One. However, the idea of the Secret Fire being a possible link to the concept of the Holy Spirit is a good one - perhaps a theological concept expressed through an elemental one, which I like.

As there are differing concepts within Christianity of the nature of God, I find it interesting that the Catholic nature of God does not come through so clearly as it might be expected to, considering Tolkien's strong faith. We have to interpret the nature of Eru, while in Catholicism, the nature of God is central. Eru could as easily be One, as in other branches of Christianity, and indeed, in other religions.

Although, to further muddy the waters, what was Gandalf's true nature following his return to Middle Earth? Could he have been seen as a Messianic figure?

What's quite odd about this whole discussion is that as lmp said:

Tolkien asserted that the emblems of religion don't belong in fantasy

And yet, here we all are, looking for them. Would we look for them if we knew nothing of Tolkien's faith? Does it necessarily follow that because he personally was a Catholic that he would put his own religion in his work? If he made the statement that emblems of religion did not belong, then wouldn't he have taken great care that they were not there in the text? So when we find them it could be either that we want to find them, or maybe that such emblems of religion are unavoidable as they are shared across cultures and beliefs, and even across time.