"Secret Fire" in The Silmarillion

It's written in The Silmarillion:

"Then the themes of Ilúvatar shall be played aright, and take Being in the moment of their utterance, for all shall then understand fully his intent in their part, and each shall know the comprehension of each, and Ilúvatar shall give to their thoughts the secret fire, being well pleased."

Why "secret fire" is written here using only small or lower case letters? After all, in other passages the term is written "Secret Fire" or "Flame Imperishable", always with upper case letters... These last manners look more suitable, given that this entity appears to be divine in some way (something like the Holy Spirit, perhaps).