In his "Letter from Birmingham Jail," Dr. Martin Luther King Jr. references St. Augustine to emphasize the importance of justice and the moral obligation to disobey unjust laws. St. Augustine famously stated that "an unjust law is no law at all," which supports King's argument that individuals have a responsibility to challenge and resist laws that perpetuate injustice and inequality.
This type of reference is called an allusion. Allusions are indirect references to well-known figures, events, or texts, and they are often used to draw on the authority or implications of that reference to strengthen one's argument. By invoking St. Augustine, King not only aligns himself with a respected philosophical and theological tradition but also reinforces his moral stance against segregation and discrimination in a way that resonates with his audience.