The phrase "an unjust law is no law at all" is attributed to St. Augustine. St. Augustine was an early Christian theologian who lived between 354 CE and 430 CE.
The phrase is associated with natural law. Natural law assumes that there are rights that are inherent to all human beings by nature of being human. This philosophy has had a significant impact on law in the United States. The idea that there are rights that all people deserve is a founding principle in the Declaration of Independence and the Constitution of the United States.
Martin Luther King, Jr., a civil rights activist, used this phrase in his Letter from Birmingham Jail. King fought for equal protection for all ethnicities. He used this statement to explain his disobedience to unjust laws.