What is the central idea of the poem first they came by martin niemoller

Niemoller talks about how Nazis came for particular groups like the Jews, socialists, communists and trade unionists. He further says in each stanza how nobody, including himself, stood up for these groups because of their fear of the Nazis. Niemoller says how he "did not speak out because I was not ... " Jew, communist, etc. The central idea is that the German and Nazi parties came for every social group, even down to Niemoller himself for opposing the Nazis, and alas, nobody stood up for him either.