Martin Luther King begins his speech by recalling Abraham Lincoln's Emancipation Proclamation of one hundred years earlier. In this proclamation, Lincoln declared that all slaves from rebellious states were free. By citing Lincoln's proclamation of 1863, King implies that the treatment of African Americans in 1963 is in direct contravention of Lincoln's promise. King's argument is thus that African Americans in 1963 should be granted equal rights to honor the promise made by Abraham Lincoln, "a great American," one hundred years ago.
King also argues in his speech that America cannot rightly call itself a great democracy while it refuses to grant basic democratic rights to so many of its citizens. He tells his audience that "Now is the time to make real the promises of democracy." King further argues that America can not be a peaceful, prosperous nation "until the Negro is granted his citizenship rights."
King's speech was so effective in large part of course because of its central, enduring calls for justice and freedom. It was also very effective because of King's talent as an orator, and because of the many rhetorical flourishes throughout the speech. King uses lots of vivid metaphors, for example. He talks about "the quick sands of racial injustice" and about the promise of "an oasis of freedom and justice." King also uses lots of repetition to emphasize his most important points. He repeats, for example, the phrase "Now is the time" to impress upon his audience the urgency of the need for change. He also repeats the phrase "we will not be satisfied until," or a close variation of it, to warn his audience against complacency and resignation.