Frederick Douglass and the great truth of America.
That speech is absolute proof of Douglass' skills as both a writer and an orator. He knew his personal voice, he knew his audience well, and he used both to his advantage.
Douglass was borrowing directly from Isaiah: "I can't stand your New Moons, Sabbaths, and other feast days; I can't stand the evil you do in your holy meetings. I hate your New Moon feasts and your other yearly feasts. They have become a heavy weight on me, and I am tired of carrying it."