Here are some quotes from “Brotherhood”, with verse numbers:-
8……. No greater concept can be named, none which could so crown human relations and correspond to the essential nature of the Subtle and Fiery Worlds. Therefore the Brotherhood is called threefold. It extends between the three worlds as a firm bridge. It is almost impossible to imagine the contact of the earthly with the Fiery World, but under the panoply of Brotherhood such confluence is made possible.