Letter from a Birmingham Jail is an extended argument about the relationship between law and justice, the difference between those two terms and the duties to resist and change unjust laws. It is a call to action: he takes to tasks his fellow pastors and white moderates who are wringing their hands instead of joining him and his peers in the fight to change unjust laws and practices.
He helps moderns understand Plato’s Crito about when, perhaps, one should choose solidarity over individual right, and when you should choose the latter, and why time and patience can be oppression’s most powerful allies.
Much gets emphasized about MLK on his birthday: he gets quoted out of context, then we have lots of fights about how people have tried to defang him when he was really a revolutionary who stood not just for justice in civil rights, but also against poverty. I’m sure those worried about his cooptation are right.
One thing that gets lost sometimes, however, in that back and forth, is his sheer brilliance. “Letter from” is a masterpiece of composition and political thought that deserves a place in any class on justice theory.
I don’t know what else to say about Dr. King other than, like lots of Black people destroyed by white violence, he should have had the rest of his time.