Here's what one source says about Dr. King's letter:
The Letter from Birmingham Jail or Letter from Birmingham City Jail, is an open letter written on April 16, 1963, by Martin Luther King, Jr., and American civil rights leader. King wrote the letter from the city jail in Birmingham, Alabama, where he was confined after being arrested for his part in a non-violent protest conducted against segregation.
King's letter is a response to a statement made by eight white Alabama clergymen on April 12, 1963, titled "A Call for Unity". The clergymen agreed that social injustices existed but argued that the battle against racial segregation should be fought solely in the courts, not in the streets. King responded that without nonviolent forceful direct actions such as his, true civil rights could never be achieved. As he put it, "This 'Wait' has almost always meant 'Never.'" He asserted that not only was civil disobedience justified in the face of unjust laws, but that "one has a moral responsibility to disobey unjust laws." ...
The letter includes the famous statement "Injustice anywhere is a threat to justice everywhere," as well as the words attributed to William E. Gladstone quoted by King: "[J]ustice too long delayed is justice denied."
I think it's fair to say that King's letter is one of the most important and eloquent documents of the 20th Century. And it's incredible to think that King actually wrote it in a jail cell. (It's difficult for me to not also think about another great Christian leader who wrote amazing letters from jail cells.)
Read the "Call for Unity."
Then read King's "Letter from Birmingham Jail."