In his "Letter from Birmingham Jail," King takes special aim at white pastors who say they support civil rights but then attack King and his movement for not behaving in a more genteel way. In this quote, King is reiterating a theme he emphasizes throughout the letter. He is stating that the wishy washy acceptance he receives from supposedly sympathetic white pastors is more confusing that those racists who say they hate black people. When King states that the pastors' behavior is confusing, he primarily means it confuses the outside world watching the civil rights movement unfold.
When white pastors who profess to be on the side of civl rights condemn King's nonviolent action as wrong, it confuses white people who look up to these pastors as authority figures. King says it does no good whatsoever to say you support equality for black people if you don't support the efforts black people have to make to achieve that equality. King argues that some white pastors want civil rights to come without any pain or inconvenience, but he asserts that this is an unrealistic fantasy: black people will not gain anything substantive until they inflict real pain on white people in non-violent ways, such as street protests and boycotts.
King is telling white people that nothing can be accomplished without some risks, disruption, and discomfort and that to be lukewarm about this necessity is to do a very great disservice to the cause. He is drawing a line in the sand: either back his movement whole-heartedly or join the racists.
This quote from Martin Luther King Jr. is a very profound quote about the nature of convicting belief. It is an interpretation and exposition on a Bible verse in the introductory chapters of the book of Revelation where John is addressing various churches in the first century, and he says that God will "spit them out of his mouth" if they are lukewarm in their affections toward him.
The idea that drives this quotation is that true, convicting belief drives one to be anything but lukewarm. If one truly believes in something, and if that thing is worthy of action and passion, it will stir up activism and passion within that individual. If someone claims to believe in something worthwhile but can be nonchalant and passive toward it, it reveals that they truly do not care about it but are feigning belief and therefore tainting the cause with their lack of passion and concern, making it worse even than opposition or rejection of the cause or belief.
This is a powerful reflection on how racism can sometimes be concealed in society in a way that is very difficult for those who are discriminated against. If somebody rejects you outright, through the use of racist language, ostracism, and sanctions, at least you know where you stand with them—they are a racist, they will not accept you, and while this is very upsetting, at least it is clear. On the other hand, when someone exhibits "lukewarm acceptance" towards you—never using racial slurs; allowing you to use the same facilities; offering employment; and yet never fully accepting you or behaving in a friendly way, things can be much more confusing and difficult.
In a situation such as this one, it is easy to think, "I may be wrong about them—I only have to try a bit harder, and eventually they will treat me in the way that they treat everyone else. After all, this person has never been overtly racist." This puts the onus on the discriminated-against person to pursue the affections and good opinion of the other person—who, if a covert racist, will never warm towards the sufferer, no matter what they do. It muddies the waters and makes it hard to know what to do.
In King's situation, these people who offer "lukewarm acceptance" pay lip service to wanting to end racism, ostensibly supporting the rights of black people, but their actions do not match what they say (or do not say). This is confusing, because if you are not a racist, why would you stand idly by and allow racism to continue?
This is a quote from Martin Luther King's "A Letter From Birmingham Jail." I would say that this quote is not so much about racism as about fighting against it.
In this quote, King is saying that he cannot understand people who do not really care much about the issue of racism. He says he can understand it if people are racist. Those people are embracing the evil of racism and probably think it is good. But people who are only lukewarm against racism are strange to him. He wonders how you can say "well, that's wrong, but I don't really care enough to do anything about it."
So if you're looking for a paraphrase, what he's saying is "I understand if you hate me, but if you don't hate me, how can you not support me when I fight against hate?"
Like I say, this isn't really a quote about the nature of racism -- it's about the fight against it.