During this weekend's massive assault of MLK stories there was one on NPR discussing the way in which he constructed the famous "I Have a Dream" speech. The speech (both the written first part and the digression of the second half) was really a sermon that had been developed over time as he went about his ministry. Many times this fact is glossed over (as it is a bit here by the Mahalia Jackson story/quote "tell them about the dream, Martin" which, of course, actually implies that she had heard it before).
The speech has become--rightly--an important and defining moment in the history of the country. This, I think, has made folks want to elevate it to some magical plane. Of course, in real life, MLK was a great and gifted pastor doing what most pastors do every week. I commend the first part of the article to you for this reason. That is, not because it is unusual, but because it is a nice summary of the mainline or liberal preacher's art as it is practiced every week.
Good preachers preach in the language of their listeners. The purpose isn't to be confusing or to demonstrate our own cleverness but instead to be invitational. The preacher welcomes (or can welcome) the listener into the text and context of the sermon. At the Lincoln Memorial, MLK nailed it as he had often done before.
Here is a transcript and recording of the report