Silmarillion - Ainulindale

[mark12_30]

We are never told what exactly this is, though it appears to be some kind of creative force, and it is said to be “with Iluvatar”.

Clyde S. Kilby notes in his book "Tolkien as Christian Writer" that:
Quote:
Professor Tolkien talked to me at some length about the use of the word "holy" in The Silmarillion. Very specifically he told me that the "Secret Fire sent to burn at the heart of the world" in the beginning was the Holy Spirit

In terms of Middle-earth, what is 'the heart of the world'? Where is it?