Dr. King uses an allusion when he refers to the Reverend Reinhold Niebuhr, an American theologian and professor as well as a leading intellectual during the mid-20th century. Niebuhr wrote several influential books, one in particular that addressed the morality of the individual versus the immorality of society. He is also credited with writing the Serenity Prayer. Dr. King writes,
History is the long and tragic story of the fact that privileged groups seldom give up their privileges voluntarily. Individuals may see the moral light and voluntarily give up their unjust posture; but, as Reinhold Niebuhr has reminded us, groups are more immoral than individuals.
This reference to such an eminent person—one who has actually been called the greatest Protestant theologian in the last two centuries—strengthens Dr. King's argument and credibility with his intended audience, a group of white, Southern religious leaders. Not only is he familiar with Niebuhr's work, but he is also influenced by it, as they likely have been as well.
Dr. King employs another allusion when he refers to the common drug, thalidomide. He says that black Americans are often told to wait, as though waiting will eventually result in there being a good time, so to speak, for them to demand their rights. However,
This 'wait' has almost always meant 'never.' It has been a tranquilizing thalidomide, relieving the emotional stress for a moment, only to give birth to an ill-formed infant of frustration.
Thalidomide was a drug used to treat anxiety and other kinds of tension as well as morning sickness in gestating women. Sadly, thalidomide causes terrible birth defects and many thousands of infants were born with malformed limbs, among other problems, as a result of their mothers' ingestion of the drug while pregnant. Many of these infants could not survive. Here, Dr. King continues to show his own intellectual capacity and the extent of his knowledge, referencing what would have been a very well known issue of the time, something sure to grab the attention of his audience and give them pause, make them think about how the black community has been placated only to have even bigger problems later.
Dr. King employs another set of allusions when he writes that
You express a great deal of anxiety over our willingness to break laws [...]. Since we so diligently urge people to obey the Supreme Court's decision of 1954 outlawing segregation in the public schools, it is rather strange and paradoxical to find us consciously breaking laws. One may well ask, "How can you advocate breaking some laws and obeying others?" The answer is found in the fact that there are two types of laws: there are just laws, and there are unjust laws. I would agree with St. Augustine that "An unjust law is no law at all."
Here, he alludes to the Supreme Court ruling in favor of desegregation, Brown v. Board of Education. The highest court in the land has ruled that desegregation of schools must take place but a black person still cannot sit at the same coffee counter as a white person? He also alludes to St. Augustine, another important touchstone for the religious community. In making these references, Dr. King both establishes his knowledge of the law as well as his knowledge of what is right. The idea that one does not have to obey an unjust law is not his own but is grounded in religious authority and sacred texts.