On the eve of Nelson Mandela’s 100th birthday anniversary, 15,000 people—including South Africa’s President Cyril Ramaphosa and Mandela’s widow, Graça Machel—were thrilled by a speech that not only paid tribute to Madiba’s memory but called South Africans to new hope and political conversion. Live-streamed before millions more, the speaker was former United States President Barack Obama.

Clearly buoyed by the multiracial crowd that frequently interrupted him with resounding applause, Mr. Obama gave what some commentators consider his most important speech since he vacated the Oval Office in January 2017. With passion and reason, Mr. Obama noted that in the century since Mandela’s birth humanity had developed a new and inclusive vision, despite all attempts, past and present, to roll it back.

“More and more peoples,” he said, “having witnessed the horrors of totalitarianism, the repeated mass slaughters of the 20th century, began to embrace a new vision of humanity, a new idea, one based not only on the principle of national self-determination but also on the principles of democracy and rule of law and civil rights and the inherent dignity of every single individual.”

Obama gave what some commentators consider his most important speech since he vacated the Oval Office.

Despite terrible setbacks, he continued, these values have become integral to humanity’s sense of self from the local to the global level, not simply in the embrace of democracy as the norm but in economic terms: not simply market-based economies in an open global system but also principles of welfare and labor rights. This progress is also seen in the rejection of discrimination against persons based on race, religion and gender.

He noted, too, the problems humankind faces at this time. Discrimination is still real. Inequality persists. “A few dozen individuals,” he said, “control the same amount of wealth as the poorest half of humanity. That’s not an exaggeration; that’s a statistic.” Far-right business interests use economic clout to manipulate elections, drawing on populism to generate a “politics of fear and resentment” that seeks to undermine a century of democratic gains.

Though he never mentioned his successor by name, he noted, “Strongman politics are ascendant suddenly, whereby elections and some pretense of democracy are maintained—the form of it—but those in power seek to undermine every institution or norm that gives democracy meaning.”

“Unfortunately, too much of politics today seems to reject the very concept of objective truth. People just make stuff up.”

Later he remarked: “Unfortunately, too much of politics today seems to reject the very concept of objective truth. People just make stuff up. They just make stuff up.

“We see it in state-sponsored propaganda; we see it in internet-driven fabrications; we see it in the blurring of lines between news and entertainment; we see it in the utter loss of shame among political leaders where they’re caught in a lie and they just double down and they lie some more.”

Everyone knew he was alluding to President Trump. For South Africans, it could also have applied to former President Jacob Zuma.

In contrast, Mr. Obama stated what he believed in: “I believe in Nelson Mandela’s vision…a vision shared by [Mahatma] Gandhi, [Martin Luther] King and Abraham Lincoln…a vision of equality and justice and freedom and multi-racial democracy, built on the premise that all people are created equal, and they’re endowed by our creator with certain inalienable rights.”

“We have to follow Madiba’s example of persistence and hope.”

Mandela’s vision, he concluded, offers a few “guideposts” for the current struggle for a more just and inclusive future.

First: “Those of us who believe in freedom and democracy…have to fight harder to reduce inequality and promote lasting economic opportunity for all.” Mandela reminds us, he said, “that some principles really are universal—and the most important one is…that we are bound together by a common humanity and that each individual has inherent dignity and worth.” He reminds us that “democracy is about more than just elections.”

Finally, Mr. Obama said, “We have to follow Madiba’s example of persistence and hope.”

As I watched Mr. Obama, the historian in me recalled another speech by a U.S. public figure in South Africa, way back in 1966: Robert F. Kennedy. To my delight, Mr. Obama alluded to that historical moment in his speech. It reminded me that the problems faced by South Africa mirror the problems of the United States. In Barack Obama, I sensed someone who understood this, too.

The vision of inclusion, tolerance and dialogue—what we in Africa call Ubuntu—that permeated Mr. Obama’s talk recalled to many of us Nelson Mandela at his best. In this diminished political era, we saw true statesmanship again, and the chilly South African Highveld winter was warmed by a bit of the old Madiba magic that we have missed for so long.

Anthony Egan, S.J., contributes from Johannesburg.