The United Methodist Church is suing one of its largest and wealthiest congregations, pitting a shrinking mainline denomination against a powerful individual church in a clash over who controls the congregation’s future.

Highland Park United Methodist Church is what counts for “old money” in Dallas, a cheerfully capitalist city where new money tends to be welcomed without much fuss. George W. Bush is a longtime member, and Ross Perot’s funeral was held in its Gothic-style sanctuary. More than 19,000 people attended Easter services there this year.

The church’s growth and financial health place it in contrast with the United Methodist Church, which it has belonged to since the congregation was established on the campus of Southern Methodist University in 1916. In a yearslong schism primarily over L.G.B.T.Q. issues, about 7,600 mostly conservative churches around the country departed the denomination between 2019 and 2023, including at least 40 percent of the churches in Texas.

Highland Park opted to stay, but it quietly revised its bylaws in 2022, a period in which no one was sure which side would take control of the denomination. The new bylaws granted Highland Park the power to make decisions typically made by the denomination, such as hiring clergy. They also removed all mentions of the United Methodist Church and the Book of Discipline, its governing text.

The conflict reflects something broader going on in American religion:​ Mainline denominations like the United Methodist Church​ are declining in both members and influence,​ making denominational identity less powerful than it was once was​. The effect is that denominational leadership has lost some of its leverage over individual churches, especially the largest ones.

What leverage the national leadership does have is often ultimately based in a denomination’s governing texts. The United Methodist “trust clause” means that if an individual church leaves, the denomination retains ownership of its property, for example.

Highland Park’s changes to its bylaws went too far, according to the suit filed last month in a Dallas civil court by the Horizon Texas Annual Conference, a regional body that oversees United Methodist congregations in North Texas and the state’s panhandle.

The lawsuit accuses the church of quietly severing important legal ties to the denomination while continuing to present itself as a United Methodist institution.

“Since last fall, conference leaders have sought to resolve this directly with Highland Park’s leadership, but that request has not been accepted,” Bishop Ruben Saenz Jr., the leader of the conference, said in a video statement after the suit was filed.

The church responded fiercely, calling the lawsuit “unnecessary, unwarranted and unjust” and disputing many of its claims. Its leadership council also unanimously passed a resolution declaring that while it considers the conflict, it no longer recognizes the authority of the bishop.

A spokeswoman for the church said in a lengthy statement that the changes were made to protect it legally during a time of “significant uncertainty” in the denomination, and that Highland Park had no intention of leaving “until the conference filed this unnecessary and unfortunate lawsuit.”

“We simply followed the best advice we could from courts to keep our corporate documents corporate and enforceable under neutral principles of Texas law,” the statement says.

It also refers to a 2020 Texas Supreme Court decision in which the court allowed a breakaway diocese in Fort Worth to retain its $100 million worth of property even after splitting with the Episcopal Church.

Highland Park’s senior minister, Matt Tuggle, told the congregation at a recent meeting that he was personally under disciplinary review by the conference, in part for undermining the bishop’s authority.

Highland Park’s refusal to acquiesce has confused observers who know the church well.

“Everyone is having difficulty understanding it,” said George Mason, who sits on the board at Southern Methodist University’s school of theology with Bishop Saenz and Mr. Tuggle.

He pointed out that Highland Park has given millions to the denomination and to other Methodist congregations with fewer resources, embodying the “connectional” spirit of Methodism. But he also characterized the church’s push for greater independence as part of a larger American phenomenon in which most churchgoers ultimately want decisions to be made at a local level.

“If you’re a church like Highland Park, you are a denomination unto yourself,” he said. “You don’t have to depend on the denomination for resources.”

At the standing-room-only meeting for church members on Aug. 26, which was not streamed online, Mr. Tuggle depicted the lawsuit as shocking and insulting. He also characterized the church’s current bylaws as a simple protective measure.

“If there is ever a time where there is some kind of overreach,” he said, “we need a safety net.”

Southern Methodist University, with which Highland Park retains close ties, attempted to extricate itself from the United Methodist Church in 2025. That dispute was settled this year, and the university remains connected with the denomination.

Whatever the outcome of Highland Park’s clash with the United Methodist Church, Mr. Tuggle said at the meeting, “we are going to continue to be a great Wesleyan Methodist church.”

It was an assurance that cited the church’s theological heritage — but not its denomination.