What was Martin Luther King Jr's profession, and how did it play a role in the tone of the Letter from a Birmingham Jail?

Martin Luther King Jr's was Minister and activist, and the letter from a Birmingham was a campaign written on April 16, 1963, and was an important text in the American Civil Rights Movement, and the letter defends the strategy about the no violence against the racist, and he written a paragraph where the injustice is a threat to justice.