History has proven that inner determination can often break through the outer shackles of circumstance. Take the Jews for example. For years they have been forced to walk through the dark night of oppression. They have been carried through the fires of affliction, and put to the cruel sword of persecution. But this did not keep them from rising up with creative genius to plunge against cloud-filled nights of affliction, new and blazing stars of inspiration. Being a Jew did not keep Spinoza from rising from a poverty stricken ghetto to a place of eminence in philosophy. Being a Jew did not keep Handel from lifting his vision to high heaven and emerging with creative and melodious music that still shakes the very fiber of men's souls. Being a Jew did not keep Einstein from using his profound and genius-packed mind to challenge an axiom and add to the lofty insights of science a theory of relativity...Whoa, whoa, back up. Handel was Jewish? Somebody tell Michael Marissen!
As far as I can tell, that "plunge against cloud-filled nights of affliction" phrase was King's own, but it sure sounds like a quote. King liked it enough to use it in other speeches throughout his career, including his 1961 "The American Dream" commencement address at Lincoln University.