Answer:
The rhetorical device Dr. Martin Luther King is using in this excerpt from his "Letter from a Birmingham Jail" is Antithesis. This device involves the juxtaposition of contrasting ideas, often in parallel structure, to emphasize their differences or contradictions. In this case, King contrasts the ideas of sitting idly by in Atlanta with being concerned about what happens in Birmingham, as well as the concepts of injustice anywhere and justice everywhere. This sharp contrast serves to emphasize his main point about the universality of justice.