President Biden, reeling from a disastrous debate performance and calls to step aside, addressed a Black church service in Pennsylvania on Sunday, acknowledging the "world's looking to America."

Speaking from a stage at Mount Airy Church of God in Christ in northwest Philadelphia, the 81-year-old Biden laughed off concerns about his age, joking, "I know I look 40" but "I've been doing this a long time."

"I, honest to God, have never been more optimistic about America's future if we stick together," Biden said.

The president, later on in his remarks, also addressed the upcoming NATO summit in Washington, D.C.

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President Biden speaks at a church service at Mt. Airy Church of God in Christ, Sunday, July 7, 2024, in Philadelphia. (AP Photo/Manuel Balce Ceneta)

"I'm about to host the NATO nations in Washington. We put them together," Biden said. "The world's looking to us. Not a joke. The world is looking to America not to carry their burden, but to lead their hopes." 

"When I ran for the first time for president, I said something basic. I said, we have to bring back dignity and hope in America, number one," the president added, wrapping up his remarks. 

"Number two, we have to give working class and middle class people, like the family I came from, a shot and build the economy from the middle out and the bottom up, not the top down," Biden said. "And thirdly, we must unite America again. That's my goal. That's what we're going to do. God bless you all and may God bless our troops." 

NATO Secretary-General Jens Stoltenberg was asked during an appearance on CBS’ "Face The Nation" about whether Sen. Lindsey Graham, R-S.C., was accurate in voicing concerns that world leaders don’t trust Biden to be in command of the job. 

President Biden and Bishop Ernest C. Morris Sr. during a church service at Mt. Airy Church of God in Christ, Sunday, July 7, 2024, in Philadelphia. (AP Photo/Manuel Balce Ceneta)

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"I think it's important for NATO to stay out of that kind of domestic discussion," Stoltenberg said. "They're of course important for the United States, but NATO should not be part of it. What matters for NATO is the decisions. What to do together. And just for instance, on defense spending, which has been a big issue for the United States for many years under different presidents. When we made the pledge 10 years ago to increase defense spending, only three allies spent 2% of GDP on defense. This year, it’s 23 allies." 

NATO Secretary-General Jens Stoltenberg speaks with President Biden during a meeting of the NATO-Ukraine Council in Vilnius, Lithuania, July 12, 2023. (AP Photo/Pavel Golovkin, File)

Biden and his NATO counterparts are meeting in Washington this week to mark the 75th anniversary of the world’s biggest security organization just as Russia presses its advantage on the battlefield in Ukraine.

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The three-day summit, which begins Tuesday, will focus on ways to reassure Ukraine of NATO’s enduring support and offer some hope to its war-weary citizens that their country might survive the biggest land conflict in Europe in decades. NATO’s day-to-day work is led by Stoltenberg, the former prime minister of Norway, until he is replaced as secretary-general on Oct. 1 by outgoing Dutch Prime Minister Mark Rutte.

The Associated Press contributed to this report.