Which was a key message of martin luther king, jr.'s "letter from birmingham jail"?

The Letter from Birmingham Jail was an open letter written on April 16, 1963 by Martin Luther King, Jr., one of the leaders of the Civil Rights Movement in the United States. King wrote the letter from the prison in the city of Birmingham in Alabama, where he was being held after a non-violent protest against racial segregation. The letter is a response to a statement issued by eight white clergy members of Alabama on April 12, 1963 entitled "A Call to Unity." In it, they declared the existence of social injustices but expressed the belief that the battle against racial segregation should be carried out only in the courts and not carried to the streets. King responded that without strong direct action, like his, true civil rights could never be achieved. As he said "This 'Wait' almost always wanted to say 'Never'".  In it, King established not only that civil disobedience was justified in the face of unfair laws, but that "one has the moral responsibility to disobey unfair laws."

The letter includes a quote that is frequently used: "Injustice anywhere is a threat to justice everywhere," as well as the words quoted by King of Thurgood Marshall: "Justice too long delayed is justice denied."

where is the subject? hahahaha