The Secret Fire
Flame Imperishable; the fire of Eru that is the heart of being
Definition
The Secret Fire — also the Flame Imperishable — is the principle of life and reality that belongs to Eru Ilúvatar alone: “the Fire that gives Life and Reality to all the world”. It is not a fire of burning but the fount of being itself — that by which the Ainur were created out of the thought of Ilúvatar, and by which the world was “filled” at its making.
In the Ainulindalë
The Ainur - — “they were the children of his thought, and he made them to live by the Secret Fire”. At the world’s making Eru declared: “And I will send forth into the Void the Flame Imperishable, and it shall be at the heart of the World, and the World shall Be.” The world was thus kindled from within; its reality is the hidden Fire.
Meaning
The Secret Fire is Tolkien’s rendering of the presence of God in creation — the “luce viva” — the doctrine that all being is held in being by its creator, and that no creature can create but only subcreate. It is why the Valar shape but do not make life; why Aulë could not give the Dwarves life until Eru “hallowed them”; why Sauron and Morgoth, however mighty, rule only by theft and domination. Its absence explains the horror of the undead and of the empty things that Sauron animates: mere masks of will.
In the Texts
Chiefly the Ainulindalë and the discussions in Morgoth’s Ring (Tolkien explicitly associated it with the Holy Spirit in his later letters and notes). Christopher Tolkien prints the relevant authorial glosses, tracing the term from the early to the final form.