At the beginning of the letter, Martin Luther King makes his points with brevity and elegance when he employs double antithesis in short, gnomic phrases:

Injustice anywhere is a threat to justice everywhere.

Here, in a sentence of eight words, King has the antithesis between injustice and justice, as well as the contrast between anywhere and everywhere. The grammatical parallelism heightens the rhetorical effect. He repeats the structure in another memorable phrase, which sounds like a proverb:

Whatever affects one directly, affects all indirectly.

King later uses the same dual structure as an elegant way to conclude longer sentences, as when he observes:

We know through painful experience that freedom is never voluntarily given by the oppressor; it must be demanded by the oppressed.

The use of antithesis is frequently varied throughout the letter, exhibiting King's mastery of pace and rhetoric. The three examples above are all dual antitheses. These give a stately balance to the phrase, reminiscent of the great prose stylists of the eighteenth century. However, a single antithesis is pithier, and can be expressed in fewer words, such as the final four of this sentence:

Too long has our beloved Southland been bogged down in a tragic effort to live in monologue rather than dialogue.

Sometimes, however, King takes the opposite approach, expanding his sentence into a long, drawn-out antithesis or series of antitheses:

Thus it is that I can urge men to obey the 1954 decision of the Supreme Court, for it is morally right; and I can urge them to disobey segregation ordinances, for they are morally wrong.

Here we have three antitheses in the same sentence: obedience and disobedience, the two different laws and, finally and most powerfully, the simple contrast between right and wrong. There are many more examples, including King's frequent contrasts between justice and injustice, often in parallel sentences:

One has not only a legal but a moral responsibility to obey just laws. Conversely, one has a moral responsibility to disobey unjust laws.

The continual use of antithesis to highlight moral contrast is one of the most effective devices King employs in the letter, creating many memorable phrases still quoted by activists in a range of causes today.

Antithesis, literally meaning "opposite," is the juxtaposition of two contrasting ideas within a parallel grammatical structure. Indeed, literally, to use antithesis is to posit two theses (ideas) that seem to oppose each other: anti-theses. When used as a rhetorical device, antithesis employs similar grammatical structures to draw attention to, and thus amplify, the contrast between the two theses.

In King's letter, written during his incarceration in Birmingham Jail at the time of the 1963 Birmingham Campaign, antithesis is used to express King's key concern that there is one rule in America for whites, and another for blacks, with King's peaceful protest being met with harsh rebukes. Indeed, because King felt that treatment of blacks ran antithetical to that of whites, the rhetorical device suits his purpose well. For example: "freedom is never voluntarily given by the oppressor; it must be demanded by the oppressed." The antithesis here is very much marked, with "given" opposing "demanded," while "oppressor" counteracts "oppressed."

Another example in the same vein is "justice too long delayed is justice denied." The antithesis here serves to underline the hypocrisy inherent in the system: in King's view, to "delay" justice is no better than to deny it outright, as delaying justice is an injustice in itself.

We can also see some examples of extended parallelism in this letter, utilising anaphora (repetition of the same word or phrase at the beginning of several sentences or clauses within sentences) to amplify the antithetical effect. For example,

So I, along with several members of my staff, am here because I was invited here. I am here because I have organizational ties here.

But more basically, I am in Birmingham because injustice is here.

Here, the paragraph break serves as a caesura or dramatic pause, hugely amplifying the effect of the antithesis as it illuminates how the "basic" reason seems to contrast to the other, more mundane ones that precede it.

Antithesis is the opposition or contrast of ideas or words in parallel structure.  Parallelism occurs when structures within sentences or parts of a sentence take the same form.  Parallelism is a grammatical repetition.

One of King's most quoted lines is an example of antithesis: "Injustice anywhere is a threat to justice everywhere."  First, one must understand how this sentence is in parallel form.  If one were to break the sentence down to its parts of speech, it would follow this structure: noun, adverb, verb, adjective, noun, preposition, noun, adverb.  The two parts that are underlined show the two structures that are repeated, so one knows this is an example of parallelism.  Looking at those two parts in the sentence ("Injustice anywhere" and "justice everywhere"), one can see that the phrases are opposites of one another.

So what is the reason for antithesis?  King wants to show his audience (particularly the eight clergymen who sent him the letter to which he is responding) that his purpose in being in Birmingham is greater than Birmingham; it is about rooting out injustices across the country to protect and elevate justice for all.  It is not a Black or White issue; it is the country's issue.

Some other examples of antithesis (with the parallelism underlined):

1. "Whatever affects one directly, affects all indirectly." (para. 5)
2. "...the kind of tension in society that will help men rise from the dark depths of prejudice and racism to the majestic heights of understanding and brotherhood." (para. 10)
3. "We know through painful experience that freedom is never voluntarily given by the oppressor; it must be demanded by the oppressed." (para. 13)
4. "One has not only a legal but a moral responsibility to obey just laws. Conversely, one has a moral responsibility to disobey unjust laws." (para. 15)
5. "Any law that uplifts human personality is just. Any law that degrades human personality is unjust." (para. 16)
6. "An unjust law is a code that a numerical or power majority group compels a minority group to obey but does not make binding on itself. This is difference made legal. By the same token, a just law is a code that a majority compels a minority to follow and that it is willing to follow itself. This is sameness made legal." (para. 17)
7. "Lukewarm acceptance is much more bewildering than outright rejection."