President Obama joined the nation in mourning and remembrance at the Pentagon on Sunday, paying tribute to the victims, survivors, and families of the September 11 attacks.For the final time during his presidency, Obama spoke about the nation's resilience in the face of tragedy and why that strength is "the most enduring memorial to those we lost."On Sunday morning, the president observed a moment of silence in the White House to commemorate the moment when the first plane hit the north tower of the World Trade Center.During his speech at the Pentagon, the president quoted scripture, saying, "Let not steadfast love and faithfulness forsake you. Bind them around your neck. Write them on the table of your heart."As the president spoke about the ever-evolving fight against terrorism, he noted that staying true to the nation's ideals will provide strength. "It's so important today that we reaffirm our character as a nation. People draw from every corner of the world, every color, every religion...bound by a creed as old as our founder."He added, "We know that our diversity, our patchwork heritage, is not a weakness. It is still and always will be one of our greatest strengths. This is the America that was attacked that September morning. This is the America we must remain true to."The speech was an echo of his most recent weekly address , which also focused on remembering the 9/11 attacks. In that address, the president said, "We cannot give in to those who would divide us."Democratic presidential candidate Hillary Clinton, who was a New York senator in 2001 , attended the memorial service in New York and took to Twitter to pay tribute on Sunday morning."We'll never forget the horror of September 11, 2001. Today, let's honor the lives and tremendous spirit of the victims and responders," she wrote.