"But let justice roll on like a river, righteousness like a never-failing stream."
"I have a dream that one day this nation will rise up, live out the true meaning of its creed: We hold these truths to be self evident, that all men are created equal."
King references the Gettysburg Address and Abraham Lincoln because he gave his speech in front of the Lincoln Memorial, so it touches base to a time where they were still fighting for freedom and justice, and it establishes his credibility.
Martin Luther King references the Bible, more specifically, Amos, in his speech. He wants to to spread justice and equality for all races, so he references Amos so he can establish credibility through God and relate to the Biblical era.
MLK references the declaration of independence in his speech. Our founding fathers wrote in the Constitution that all men are created equal, and KING wants to keep that idea living, so he establishes credibility through the Declaration.