To whom is King writing the letter from Birmingham city jail?

King is responding to group of white Southern clergymen who'd openly criticized the civil rights movement for engaging in unlawful protest as part of its campaign against racial segregation. The clergymen claimed that they were sympathetic to the civil rights cause, but that the best way to achieve change was through exclusively legal means.
In his response, King justifies his actions on the grounds that unjust laws must be challenged, and that if the civil rights movement adopts a purely legal approach then it will never succeed, as it is the law itself that's part of the problem. He also enjoins the white clergymen as fellow Christians to understand that they have a duty to challenge injustice and repression wherever they appear, and that this will often entail defying and breaking the law.