Question: From Obama's second inaugural speech:

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> We, the people, declare today that the most evident of truths – that all of us are created equal – is the star that guides us still; just as it guided our forebears through Seneca Falls, and Selma, and Stonewall; just as it guided all those men and women, sung and unsung, who left footprints along this great Mall, to hear a preacher say that we cannot walk alone; to hear **a King** proclaim that our individual freedom is inextricably bound to the freedom of every soul on Earth.
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I know that he was referring to Martin Luther King, Jr. But why did he use the indefinite article "a" before "King"? Is it better, for example, to use "Dr. King" or just "King" instead? Does the use of the word "a King" provide a sense that Dr. King was the king of something, such as righteousness, justice, etc?

Answer: 
Stylistically, the phrase "to hear a King proclaim..." echoes the previous construction "to hear a preacher say...." The repetition of the indefinite article maintains the flow of the speech. (Imagine how awkward it would have sounded if, in place of "to hear a King proclaim," Obama had said "to hear the Reverend Dr. Martin Luther King, Jr. proclaim....") Of course, the construction also invites the listener to consider the double-meaning of "King"--as the other answers say, it identifies the speaker (MLK) and also the speaker's symbolic importance (as a leader of the Civil Rights movement and as the "moral leader of our nation").
Perhaps he meant to give the phrase a subtle *double entendre*. I could be read to mean Martin Luther King Jr. and it could also have a meaning that we need to be guided by leaders (even if they are kings) who value individual freedom for all people. 

This is perhaps a subtle, somewhat ironic reference to some kings elswhere who do not put freedom of their own subjects, let alone the freedom of all people, as their first priority.