Today, Americans are celebrating the 50th anniversary of the Rev. Martin Luther King Jr.’s “I Have a Dream” speech, which King delivered at the Lincoln Memorial on Aug. 28, 1963.

But while “I Have a Dream” is King’s most-famous speech, his most famous written work is most likely his “Letter from Birmingham Jail,” which King wrote earlier in 1963, after he had been arrested on Good Friday for marching to protest segregation in Birmingham, Ala.–in violation of a state court order prohibiting such marches.

King’s letter responded to a group of clergymen who questioned why he engaged in this type of civil disobedience.

In his explanation, King cited St. Augustine and St. Thomas Aquinas in arguing that a just law is a law that squares with the law of God, and an unjust law is one that does not.
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