1. Lincoln begins this speech by referring to how the nation began, as a county "dedicated" to the idea that all men are equal".
2. This sets the context for his discussion of the Battle of Gettysburg and the great sacrifice that the men who fought there had made. Their sacrifices had greatly contributed to the original idea of the founders of the nation.
3. Finally, he notes that those who remain must continue to fight on so that the nation founded on the premise that "all men are created equal" will not "perish from the earth."
Lincoln began by noting that the nation had been born 87 years earlier "conceived in Liberty and dedicated to the proposition that all men are created equal." The civil war, he said, was a test of whether a nation based on those principles could survive.
He then goes on to laud the sacrifice of those who had fought in the war, and said the living must dedicate themselves to giving the nation "a new birth of freedom" and ensuring that "government of the people, by the people, and for the people shall not perish from the earth."
As you can see, there is really one main issue here. Can a free and democratic nation survive? You could treat each of the principles he cites as a separate issue -- freedom, equality, and democracy.