This question already has an answer here:
Did anyone in Middle Earth (besides the Istari and other Ainur) know of or acknowledge the existence of Eru Illuvatar? Are there any temples demonstrating worship for Eru or any text from the books indicating an acknowledgement of a supreme being?
I apologise for the fact that these are two seperate questions, but it is impossible to have one without the other: One, who knew of Eru's existence? Two, who worshipped him?