King uses three other devices very effectively in his letter: biblical comparisons, rhetorical questions, and allusions.
A simile is the comparison of one thing to another thing that is not specifically like it, but which shares a quality in kind. Biblical stories are helpful here, not only for their familiarity to the eight white Southern religious leaders whom King addresses, but also to help these religious leaders better identify with a struggle that may make little sense to them.
King compares himself to "eighth-century prophets [who] left their little villages" and Paul, who "left his little village of Tarsus [to carry] the gospel of Jesus Christ to practically every hamlet and city of the Greco-Roman world." He makes this comparison to address their concern about King—a perceived outsider—coming into Birmingham from Atlanta to agitate for the cause of civil rights. In making this comparison, he likens the importance of his message to the importance of Christ's teachings—for, the equal treatment of all is, in King's estimation, a Christian message.
A rhetorical question is a question that is asked without any expectation of receiving an answer. The question may or may not have an answer. Here, King assumes what the religious leaders may have been thinking: "Why direct action, why sit-ins, marches, and so forth? Isn't negotiation a better path?" He then answers the question with an explanation of his strategy. Yet, his explanation is less sequential and technical than it is emotional and metaphorical. He explains the need to create "tension" and "dramatize the issue." He believes that, without producing such an emotive effect, a routinely ignored community can never make a strong, public impact.
In some passages, he uses a lot of figurative language. He compares the civil rights workers to Socrates and "non-violent gadflies," who create the tension that will help the South rise "from the dark depths of prejudice and racism to the majestic heights of understanding and brotherhood." Here, King uses dualistic imagery to show the contrast: the dark, low evils of "prejudice and racism" are pitted against the "majestic heights of understanding." This is also an allusion to Heaven and Hell.