President Barack Obama, marking the ninth anniversary of the Sept. 11 attacks, said Americans should remember the national unity and the “human capacity for good” that followed the nation’s worst act of terrorism.

In his weekly address on the radio and Internet, Obama said the lesson to be drawn from the attacks is that the nation is bound by “a set of common ideals” that can be exemplified by service.

“By giving back to our communities, by serving people in need, we reaffirm our ideals,” he said. “We prove that the sense of responsibility that we felt for one another was not a fleeting passion.”

This year’s commemoration of the deadliest terrorist attack in U.S. history has become entangled in controversies about plans to build an Islamic center near the site where extremists slammed two hijacked airliners into New York City’s World Trade Center (Ground Zero) and a Florida pastor’s threat to burn the Muslim holy book on the anniversary. Amidst international uproar, Pastor Terry Jones decided to cancel his plans.

Obama, in his address, said he recognized that the country is going through “a time of difficulty” and warned against divisiveness.

“It is often in such moments that some try to stoke bitterness,” he said, without mentioning the mosque site or the Florida pastor. “But on this day, we are reminded that at our best, we do not give in to this temptation.”

“It was not a religion that attacked us that September day. It was al-Qaida, a sorry bank of men which perverts religion,” Obama said Saturday at a memorial service at the Pentagon honoring the 184 government workers and airline passengers who perished when hijacked American Airlines Flight 77 slammed into the Defense Department headquarters at 9:37 a.m.

Vice President Joe Biden attended a memorial Saturday at the World Trade Center site in New York, while first lady Michelle Obama and former first lady Laura Bush spoke at a service in Shanksville, Pennsylvania, where United Airlines Flight 93 crashed as passengers tried to wrest control from hijackers.

Obama said the anniversary is a time to “renew our resolve against those who perpetrated this barbaric act of terror.”

“We will never waver in defense of this nation,” the president said. “…we will not sacrifice the liberties we cherish or hunker down behind walls of suspicion and mistrust.”