Twenty-five years ago, I was sitting at a desk on Wall Street with a career that seemed to be moving in a straight line. Sept. 11 broke that line.

Like millions of Americans, I watched the attacks with shock and anger. But what stayed with me was the uncomfortable realization that I had never seriously asked what I could be doing in moments that demanded more from all of us. So I enlisted in the Marine Corps.

The experience changed my life and, years later, influenced my decision to enter public service and be elected mayor of Jersey City.

But 25 years later, the lesson I take from Sept. 11 is not that everyone should have enlisted. It is not that the wars that followed were wise. And it is certainly not that Americans should pledge blind loyalty to their government.

The lesson was simpler and more demanding: citizenship requires so much more than voting, paying taxes, or having strong political opinions. It requires participation.

We live in an age where nearly everyone has an opinion, yet people rarely engage civically. We can announce our views instantly, condemn strangers we have never met, sign petitions without leaving the couch, and build identities around issues we oppose.

But watching democracy play out from the sidelines does not make us a better, stronger, or more unified country.

One of the most powerful memories of the period after Sept. 11 is how many Americans instinctively understood that distinction. People did not simply react to what had happened. They looked for somewhere to be useful.

Firefighters and police officers ran toward danger while everyone else was trying to escape it. Ordinary citizens waited for hours to donate blood. Americans volunteered, enlisted, checked on neighbors, and offered help.

That is not to romanticize the post-9/11 era; there were failures and ugliness too. Even amid extraordinary acts of solidarity, Muslim, Arab, and South Asian Americans were subjected to hatred and bigotry, some of which persists today.

For a brief period, however, many Americans behaved as though being part of a society meant participating in it.

Now we sort ourselves politically, geographically, and digitally. Our communities become more homogeneous.

It becomes easier to distance ourselves from our fellow citizens by viewing them as abstractions: liberals, conservatives, immigrants, socialists, MAGA voters, Wall Street, unions, activists.

Service can break through those abstractions. When you work beside someone, depend on someone, or accept responsibility for someone, these categories become less useful.

Teach in underserved schools. Restore parks. Help communities recover from natural disasters. Care for elderly Americans. Work in public health. Build infrastructure. Serve in the military.

Imagine if more young Americans spent a year working alongside people from different races, religions, regions, economic backgrounds, and political beliefs.

I have no doubt that we would be a kinder, more understanding, and more unified nation.

My children know Sept. 11 as something that happened before they were born, and I hope they never experience a day like it. I also hope they do not need a national tragedy to see the value in stepping forward when other people need them.

That was the change Sept. 11 produced in my own life.

Before that morning, I thought mostly about where my career was going. Afterward, I started asking a different question: where can I be useful?

The important thing is that we keep asking the question.

America has spent much of the past quarter-century debating what Sept. 11 meant. One lesson has been clear: we see the best of our country when our citizens participate in something beyond themselves.

Fulop is the president and CEO of the Partnership for New York City.