President Biden said in a Sunday op-ed for The Washington Post that the Jan. 6, 2021, attack on the U.S. Capitol reminds Americans that democracy “is never guaranteed.”

“We should commit to remembering Jan. 6, 2021, every year. To remember it as a day when our democracy was put to the test and prevailed. To remember that democracy — even in America — is never guaranteed,” Biden wrote for the Post.

“We should never forget it is our democracy that makes everything possible — our freedoms, our rights, our liberties, our dreams,” he added. “And that it falls to every generation of Americans to defend and protect it.”

Monday marks four years since the Jan. 6 attack. President-elect Trump has promised to grant clemency to rioters who showed up at the Capitol that day as Congress certified Biden’s 2020 election win. The president-elect has also implied that some pardons could be issued in the first few hours of his second term.

“Violent insurrectionists attacked the Capitol, threatened the lives of elected officials and assaulted brave law enforcement officers,” Biden said of the Jan. 6 attack. “We should be proud that our democracy withstood this assault. And we should be glad we will not see such a shameful attack again this year.”

“But we should not forget. We must remember the wisdom of the adage that any nation that forgets its past is doomed to repeat it. We cannot accept a repeat of what occurred four years ago,” he added.

Also Sunday, former House Speaker Nancy Pelosi (D-Calif.) said the violence of the attack on the Capitol “didn’t end that day.”

“It didn’t end that day,” Pelosi told CBS News’s Margaret Brennan on “Face the Nation.”

“As you know, [Trump] called out to these people to continue their violence, my husband being a victim of all of that, and it still — he still has injuries from that attack.”