WordNet sense 1 (an organization founded by James Leonard Farmer in 1942 to work for racial equality):

Congress of Racial Equality

WordNet sense 2 (a small group of indispensable persons or things):

nucleus, core group

WordNet sense 3 (the central meaning or theme of a speech or literary work):

burden, effect, essence, gist

WordNet sense 4 (the choicest or most essential or most vital part of some idea or experience):

center, heart, kernel, marrow, meat, nitty-gritty, nub, pith, substance, sum, inwardness, heart and soul