Answer and explanation:
An allusion is a figure of speech in which a reference is made to something or someone, but in an indirect manner. For instance, calling someone a good Samaritan is an allusion to a biblical story. The author does not explain much, since he/she assumes the audience knows whom or what he/she refers to.
In his "Letter from Birmingham Jail", Dr. King alludes to a few biblical figures. Among the ones mentioned are Shadrach, Meshach and Abednego. As we know, he alludes to them with the purpose of supporting his arguments, making his letter persuasive. He establishes a connection between his own and those biblical figures' actions when he claims that, just like them, under similar circumstances, he concluded that civil disobedience was necessary. Dr. King claims that he and those figures chose to disobey the law so that a higher moral law wouldn’t be broken. Since his audience is most likely constituted of Christians, they will grasp the message that God's law is stronger than man's law.