The promotional clip, just 1 minute 10 seconds long, begins with a cross lying atop an American flag, emerging out of the darkness.

Dramatic music rises, like a movie soundtrack for an epic battle scene, as evangelical pastors describe America as a nation originally devoted to the God of the Bible. The crescendo ends with an invitation, to an event blessed by the White House, to meet on the National Mall on May 17 to “Rededicate America as One Nation Under God.”

This weekend, thousands are expected to answer that call and join a nine-hour prayer rally that places Christianity at the center of the American story, the culmination of a yearslong effort by a powerful cohort in the MAGA movement. The event is a keystone of the Trump administration’s extensive festivities to celebrate America’s 250th anniversary.

With speeches and Christian worship performed on a Sunday against a symbolically potent backdrop at the heart of American government, the rally aims to crystallize the narrative that the nation’s founding was an intentionally Christian project, a framing disputed by many scholars. The separation of church and state has long been a bedrock principle of American democracy. The First Amendment states that “Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof.”

President Trump is expected to give a video message, and the speakers include members of the highest levels of government, including Secretary of State Marco Rubio, Secretary of Defense Pete Hegseth and House Speaker Mike Johnson. The three leaders have all stressed the priority of Christian faith in public life. A who’s who of Mr. Trump’s longtime evangelical supporters will also take the stage.

One of the speakers, the Rev. Samuel Rodriguez, president of the National Hispanic Christian Leadership Conference, said the lineup reflected the Protestant character and enthusiasms of the nation’s founding, including the Christian revivals known as the Great Awakening.

“It’s pretty much a carbon copy of what took place 250 years ago,” he said. “So it’s very honoring from an optics perspective.”

The rally, the main religious event of the Trump administration’s 250th anniversary plans, reflects the political success of a right-wing Christian movement that has intensified efforts to end the separation of church and state.

Many of the country’s founders were Christians, and references to religion are present in many of their writings. The role of Christianity in America’s founding is complex, however, and partisans have used historical data points and texts to bolster their political positions for decades.

But the idea that the founders saw America as an explicitly Christian nation is “nonsensical” and “dead wrong,” said Joseph Ellis, the Pulitzer Prize-winning historian and author of many books on America’s founding.

Instead, the founders were explicitly pushing back against an assumption that persisted through the Middle Ages, that states needed common religious preference to unite people, he said.

“It is a falsification of the meaning of the American Revolution,” he said.

In the second Trump administration, Christian worship has become routine inside the White House, and Republican power brokers have infused their public responsibilities with a sense of Christian mission. Mr. Hegseth has invoked divine purpose to justify American military power, and prayed to “King Jesus” at a White House dinner.

Mr. Johnson, who is expected to close the prayer rally, has argued that separation of church and state is “misunderstood,” and that “the Founders wanted to protect the church from an encroaching state, not the other way around.”

The prayer event again shows that the separation of church and state is “under extreme attack,” said Rachel Laser, president of Americans United for Separation of Church and State, which has filed seven lawsuits against the administration related to its turn toward Christianity.

“If Trump really wanted to celebrate what is unique and foundational about America he’d be celebrating our promise of church-state separation and religious freedom for all,” she said.

American adults largely do not want Christianity to be the country’s official religion, but over the past two years, those who do want it to be has to increased from 13 to 17 percent, according to a recent survey from the Pew Research Center.

Republicans are far more likely than Democrats to want the federal government to make Christianity the country’s official religion, the survey found.

Set against the iconic backdrop of the Washington Monument and the Capitol, the highly produced event also offers a chance for outsize reach online through conservative Christianity’s powerful social media presence.

The long-planned rally is almost entirely evangelical in character, and does not include an broad representation of Christianity or the scope of American religion. On May 4, Mr. Trump signed a proclamation calling for Jews to also observe a “national Shabbat” this same weekend.

The list of speakers includes no Muslims, at a moment when anti-Muslim hostility is on the rise in many parts of the country. The first Muslims arrived in what is now the United States before the nation’s founding via the trans-Atlantic slave trade.The event “sharpens the distinction between people who belong and people who don’t belong,” said Asma T. Uddin, an assistant professor at Michigan State University College of Law who has written about Muslims and religious freedom in the United States.

“This idea of the U.S. as Christian nation and a place that has always been fundamentally Christian erases the long histories of other religious communities in this country,” she said.

Some faith traditions are observing America’s anniversary on their own terms, without political fanfare. Next month, the U.S. Catholic bishops, who have devoted extensive efforts to standing up for immigrants during the Trump administration’s aggressive deportation campaign, will celebrate a mass in Orlando, Fla.

Cardinal Robert McElroy, the Roman Catholic archbishop of Washington, said in a recent interview that language describing America as a Christian nation “can be destructive,” especially when it’s used to center culture around the exclusion of certain groups.

Some 80 years ago, Catholics were routinely excluded in American public life, he said.

“They wouldn’t say, this is a Christian nation, they’d say, it is a Protestant nation,” he said.