The Question and Answer section for Letter From Birmingham Jail is a great resource to ask questions, find answers, and discuss the novel.
C Any type of injustice
From the text:
A just law is a man-made code that squares with the moral law or the law of God. An unjust law is a code that is out of harmony with the moral law. To put it in the terms of St. Thomas Aquinas: An unjust law is a human law that is...
In this line, King is speaking to the "true church", faith that exists beyond organized religion.