Abraham Lincoln's Gettysburg Address shifted views on the purpose of the war. Before his speech, people could have argued that the war was over states' rights vs. federal government. Or people could have argued it was a war over the issue of slavery. The Gettysburg Address changed that.
Lincoln shifted the view from those stated above to the issue of preserving the nation that was created by the founding fathers. Lincoln is calling to attention the purpose of the Declaration of Independence and all of the beliefs and values that it stood for then. Lincoln is asking the people to remember that those principles still matter. His speech is forcing people to question the validity of that government and those founding fathers. North or South, those men were respected and revered. Lincoln is suggesting that if the Union falls apart, it's equivalent to spitting on the ideas that founded the country.
The Gettysburg Address is saying that those that died there should not have died in vain if the Union is preserved. They will have died for a new birth of freedom in the United States of America.