Dr. Martin Luther King Jr. references St. Augustine in his arguments as a way to provide historical and philosophical depth to his points about justice, morality, and the struggle for civil rights. By invoking the authority of a well-respected philosopher and theologian, King enhances his argument by appealing to the audience's sense of tradition and ethical thought, thereby reinforcing the legitimacy of his stance on social justice and nonviolent resistance.
This type of reference is known as an allusion. Allusions are brief, indirect references to a person, place, thing, or idea of historical, cultural, literary, or political significance, and they can help to create connections and enrich the meaning of a text or speech. In King's case, referencing St. Augustine serves to underscore his philosophical points about love, justice, and moral responsibility.