Tolkien on "The Secret Fire"

The Secret Fire of Middle-earth

"You cannot pass," he said. The orcs stood still, and a dead silence fell. "I am a servant of the Secret Fire, wielder of the flame of Anor. You cannot pass. The dark fire will not avail you, flame of Udûn. Go back to the Shadow! You cannot pass."

—Gandalf, The Lord of the Rings, The Fellowship of the Ring

"Therefore Ilúvatar gave to their vision Being, and set it amid the Void, and the Secret Fire was sent to burn at the heart of the World; and it was called Eä."
—Valaquenta, The Silmarillion.

The following is quoted from Tolkien and the Silmarillion, by Clyde S. Kilby [via here]

Responding to a letter from Father Robert Murray suggesting Tolkien’s story impressed him as entirely about grace, Tolkien wrote: “I know exactly what you mean by the order of grace; and of course by your references to Our Lady, upon which all my own small perception of beauty both in majesty and simplicity is founded. The Lord of the Rings is of course a fundamentally religious and Catholic work; unconsciously so at first but consciously in the revision. I . . . have cut out practically all references to anything like 'religion,’ to cults and practices in the imaginary world. For the religious element is absorbed into the story and the symbolism. However that is very clumsily put, and sounds more self-important than I feel. I should chiefly be grateful for having been brought up since I was eight in a faith that has nourished me and taught me all the little that I know... “

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. [emphasis added]

He described his problem in depicting the fall of mankind near the beginning of the story. “How far we have fallen!” he exclaimed—so far, he felt, that it would seem impossible even to find an adequate prototype or to imagine the contrast between Eden and the disaster which followed.

table from "A Wind from the West: The Role of the Holy Spirit in Tolkien's Middle-Earth" by 

Gregory Hartley

image from Evan Palmer's comic adaptation of Tolkien's The Ainulindalë