What, according to Lincoln, are the responsibilities/unfinished work of the living Americans? 

Or, what is the underlying idea/ central idea/ purpose of Lincoln’s Gettysburg Address?

Abraham Lincoln (1809-1865) spoke at Gettysburg to honor the soldiers who died in the Great Civil War. He stood on the battlefield where General George G. Meade and General Robert E. Lee had fought. Lincoln spoke as the President of the Union and as the nation's moral voice. He explained that the living Americans still had important duties. He called these duties the “unfinished work.”

Must Continue the Fight for Freedom and Equality: Lincoln reminded the people that the Founding Fathers began the nation with a promise. He said, 

“All men are created equal.” 

This promise was born in 1776 when the new nation was “conceived in Liberty.” But the Civil War showed that this promise was not complete. Slavery still existed. Many soldiers died to protect this ideal. Lincoln asked the living to finish this task. Their work was to protect liberty for all people. He said the nation must have “a new birth of freedom.”

Must Honor the Brave Soldiers by Completing Their Mission: Lincoln stood at Soldiers’ National Cemetery in Gettysburg. Thousands of soldiers had died there. He said, 

“The brave men, living and dead, who struggled here, have consecrated it.” 

Their sacrifice made the land holy. Lincoln said, 

“It is for us the living to be dedicated here to the unfinished work.” 

The living must protect unity. They must protect the Union that the soldiers died for.

The Living Must Save Democracy: Lincoln warned that the nation must stay strong. He asked the people to make a promise. He said that the dead shall not have died in vain. He wanted America to protect the government 

“Of the people, by the people, for the people.” 

This was the duty of every American. This was the work that the living must complete.

Lincoln asked the living to finish the fight. Protect freedom. Honor the dead. Save democracy. Keep the nation united forever.