You can read the whole letter here. But, I realize it’s pretty long so I am providing some highlights in this post for your reading pleasure, with limited commentary.
King is a master of “practical theology,” masterfully combining beautiful imagery, theological reflection, and passionate calls to action.
On the charge that King was acting as out “outsider” in Birmingham:
Injustice anywhere is a threat to justice everywhere. We are caught in an inescapable network of mutuality, tied in a single garment of destiny. Whatever affects one directly, affects all indirectly.
On why direct action is necessary:
Nonviolent direct action seeks to create such a crisis and foster such a tension that a community which has constantly refused to negotiate is forced to confront the issue. It seeks so to dramatize the issue that it can no longer be ignored. My citing the creation of tension as part of the work of the nonviolent resister may sound rather shocking. But I must confess that I am not afraid of the word “tension.” I have earnestly opposed violent tension, but there is a type of constructive, nonviolent tension which is necessary for growth. … The purpose of our direct action program is to create a situation so crisis packed that it will inevitably open the door to negotiation.
On King’s frustration with the call to wait without action:
Perhaps it is easy for those who have never felt the stinging darts of segregation to say, “Wait.” But when you have seen vicious mobs lynch your mothers and fathers at will and drown your sisters and brothers at whim; when you have seen hate filled policemen curse, kick and even kill your black brothers and sisters; when you see the vast majority of your twenty million Negro brothers smothering in an airtight cage of poverty in the midst of an affluent society; when you suddenly find your tongue twisted and your speech stammering as you seek to explain to your six year old daughter why she can’t go to the public amusement park that has just been advertised on television, and see tears welling up in her eyes when she is told that Funtown is closed to colored children, and see ominous clouds of inferiority beginning to form in her little mental sky, and see her beginning to distort her personality by developing an unconscious bitterness toward white people; when you have to concoct an answer for a five year old son who is asking: “Daddy, why do white people treat colored people so mean?”; when you take a cross county drive and find it necessary to sleep night after night in the uncomfortable corners of your automobile because no motel will accept you; when you are humiliated day in and day out by nagging signs reading “white” and “colored”; when your first name becomes “nigger,” your middle name becomes “boy” (however old you are) and your last name becomes “John,” and your wife and mother are never given the respected title “Mrs.”; when you are harried by day and haunted by night by the fact that you are a Negro, living constantly at tiptoe stance, never quite knowing what to expect next, and are plagued with inner fears and outer resentments; when you are forever fighting a degenerating sense of “nobodiness”–then you will understand why we find it difficult to wait.
On why King advocated breaking some laws while keeping others:
One may well ask: “How can you advocate breaking some laws and obeying others?” The answer lies in the fact that there are two types of laws: just and unjust. I would be the first to advocate obeying just laws. One has not only a legal but a moral responsibility to obey just laws. Conversely, one has a moral responsibility to disobey unjust laws. I would agree with St. Augustine that “an unjust law is no law at all.”
On what makes a law just or unjust:
Now, what is the difference between the two? How does one determine whether a law is just or unjust? A just law is a man made code that squares with the moral law or the law of God. An unjust law is a code that is out of harmony with the moral law. To put it in the terms of St. Thomas Aquinas: An unjust law is a human law that is not rooted in eternal law and natural law. Any law that uplifts human personality is just. Any law that degrades human personality is unjust. All segregation statutes are unjust because segregation distorts the soul and damages the personality. It gives the segregator a false sense of superiority and the segregated a false sense of inferiority.
On the proper way to break an unjust law:
One who breaks an unjust law must do so openly, lovingly, and with a willingness to accept the penalty. I submit that an individual who breaks a law that conscience tells him is unjust, and who willingly accepts the penalty of imprisonment in order to arouse the conscience of the community over its injustice, is in reality expressing the highest respect for law.
On how complacency from people of good will is a stumbling block to justice:
I have almost reached the regrettable conclusion that the Negro’s great stumbling block in his stride toward freedom is not the White Citizen’s Counciler or the Ku Klux Klanner, but the white moderate, who is more devoted to “order” than to justice; who prefers a negative peace which is the absence of tension to a positive peace which is the presence of justice … Shallow understanding from people of good will is more frustrating than absolute misunderstanding from people of ill will. Lukewarm acceptance is much more bewildering than outright rejection.
Arguing that history is not inevitable and time is not necessarily on the side of good:
Human progress never rolls in on wheels of inevitability; it comes through the tireless efforts of men willing to be co workers with God, and without this hard work, time itself becomes an ally of the forces of social stagnation. We must use time creatively, in the knowledge that the time is always ripe to do right. Now is the time to make real the promise of democracy and transform our pending national elegy into a creative psalm of brotherhood. Now is the time to lift our national policy from the quicksand of racial injustice to the solid rock of human dignity.
Arguing against the notion that his movement is extremist:
I have tried to stand between these two forces, saying that we need emulate neither the “do nothingism” of the complacent nor the hatred and despair of the black nationalist. For there is the more excellent way of love and nonviolent protest.
Embracing the label of “extremist”:
And now this approach is being termed extremist. But though I was initially disappointed at being categorized as an extremist, as I continued to think about the matter I gradually gained a measure of satisfaction from the label. Was not Jesus an extremist for love: “Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you.” Was not Amos an extremist for justice: “Let justice roll down like waters and righteousness like an ever flowing stream.” Was not Paul an extremist for the Christian gospel: “I bear in my body the marks of the Lord Jesus.” Was not Martin Luther an extremist: “Here I stand; I cannot do otherwise, so help me God.” … So the question is not whether we will be extremists, but what kind of extremists we will be. Will we be extremists for hate or for love? Will we be extremists for the preservation of injustice or for the extension of justice?
Articulating his frustrating with the white church:
In the midst of blatant injustices inflicted upon the Negro, I have watched white churchmen stand on the sideline and mouth pious irrelevancies and sanctimonious trivialities. In the midst of a mighty struggle to rid our nation of racial and economic injustice, I have heard many ministers say: “Those are social issues, with which the gospel has no real concern.” And I have watched many churches commit themselves to a completely other worldly religion which makes a strange, un-Biblical distinction between body and soul, between the sacred and the secular.
On the courage of the early church, and the lamentable state of the modern church:
There was a time when the church was very powerful–in the time when the early Christians rejoiced at being deemed worthy to suffer for what they believed. In those days the church was not merely a thermometer that recorded the ideas and principles of popular opinion; it was a thermostat that transformed the mores of society. Whenever the early Christians entered a town, the people in power became disturbed and immediately sought to convict the Christians for being “disturbers of the peace” and “outside agitators.”‘ … Things are different now. So often the contemporary church is a weak, ineffectual voice with an uncertain sound. So often it is an archdefender of the status quo. Far from being disturbed by the presence of the church, the power structure of the average community is consoled by the church’s silent–and often even vocal–sanction of things as they are.
On King’s ultimate hope of reconciliation with his opponents, and his ultimate hope in the Civil Rights movement:
I hope this letter finds you strong in the faith. I also hope that circumstances will soon make it possible for me to meet each of you, not as an integrationist or a civil-rights leader but as a fellow clergyman and a Christian brother. Let us all hope that the dark clouds of racial prejudice will soon pass away and the deep fog of misunderstanding will be lifted from our fear drenched communities, and in some not too distant tomorrow the radiant stars of love and brotherhood will shine over our great nation with all their scintillating beauty.
 I am operating under the assumption that, since the text of the letter is available for free, in its entirety, at the link provide that I am not violating any Copyright restrictions by copying/pasting here. My sincerest apologies if I am mistaken.