Under what circumstances is extremism acceptable?

King cites that extremism is acceptable when applied to gaining an extension of justice. 

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." And John Bunyan: "I will stay in jail to the end of my days before I make a butchery of my conscience." And Abraham Lincoln: "This nation cannot survive half slave and half free." And Thomas Jefferson: "We hold these truths to be self evident, that all men are created equal . . ." 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? In that dramatic scene on Calvary's hill three men were crucified. We must never forget that all three were crucified for the same crime--the crime of extremism. Two were extremists for immorality, and thus fell below their environment. The other, Jesus Christ, was an extremist for love, truth and goodness, and thereby rose above his environment. Perhaps the South, the nation and the world are in dire need of creative extremists.