Millions of Christians in churches around the world heard the same passage on Sunday from the Gospel of Luke. In it, Jesus declares his intentions “to proclaim good news to the poor,” as he speaks to people gathered in a synagogue in Nazareth.

At Episcopal Church of the Transfiguration in Dallas, the passage felt particularly appropriate this week, though it was chosen years ago as part of a three-year cycle of Bible readings.

“It’s Jesus 101,” said Michelle Williams, 55, a parishioner at the church.

It was the first Sunday since a fellow Episcopalian, Bishop Mariann E. Budde, delivered a sermon that many observers heard as an echo of passages like the one from Luke. Speaking at a prayer service at the National Cathedral in Washington the day after President Trump’s inauguration, she faced the president and made a direct plea: “Have mercy.”

After the service, Mr. Trump called Bishop Budde a “Radical Left hard line Trump hater” in a social media post. His foes immediately hailed her as an icon of the resistance. But for many progressive Christians and their leaders, the confrontation was more than a moment of political catharsis. It was about more than Mr. Trump. It was an eloquent expression of basic Christian theology, expressed in an extraordinarily public forum.

Sara Ivey, 71, another parishioner at Church of the Transfiguration, said the sermon reminded her of Psalm 103, which describes God as “merciful and gracious, slow to anger and abounding in steadfast love.” The sermon, which she watched live, made her “very proud to be an Episcopalian,” she said.

Bishop Budde’s sermon delivered a jolt of energy in many mainline Protestant churches, whose numbers and influence have declined steeply from a high point in the middle of the last century. Some mainline Christians have sensed an unsettling whiff of irrelevance that has accelerated in the Trump era, as Mr. Trump has elevated a stream of conservative, political Christianity whose leaders in some cases do not even consider Bishop Budde a fellow Christian.

So it was startling for many progressive Christians and their leaders to see Bishop Budde’s sermon overpower the prayers that were delivered at the inauguration by clergy members who are more sympathetic to Mr. Trump — and to see her rely primarily on theological principles themselves, rather than advocating specific policies.

“A plea for mercy, a recognition of the stranger in our midst, is core to the faith,” Presiding Bishop Sean Rowe, the Episcopal Church’s top clerical leader, said in an interview. “It is radical, given the order of the world around us — it is countercultural — but it’s not bound to political ideology.”

Across the country, mainline churchgoers proudly shared clips of Bishop Budde’s sermon online and made plans to read her books with their book clubs. Priests and pastors discussed the sermon in church newsletters and in intimate conversations with their members.

The clergy members addressed it directly in their sermons, too. At Church of the Transfiguration, the associate rector, the Rev. Ted Clarkson, acknowledged to the congregation that aspects of the bishop’s sermon might have been “hard to hear.” But “mercy is truth,” he said, “and I expect a bishop to preach the truth.” (Bishop Budde preached on Sunday at a church in Maryland.)

Indeed, not all mainline churchgoers appreciated her message. Some at Church of the Transfiguration worried that it had inappropriately politicized the pulpit, or had allied the denomination with one political party, according to the Rev. R. Casey Shobe.

White mainline Christians in the United States are politically diverse, in contrast to white evangelicals, who are overwhelmingly Republican. There are still more Republicans than Democrats in mainline pews, according to an analysis of the 2022 Cooperative Election Study by Ryan Burge, a political scientist. (Episcopalians are something of an exception; 58 percent of them are Democrats.) Even so, the mainline tradition tends to be theologically liberal, preferring Bishop Budde’s message of mercy over an emphasis on judgment or authority.

Pastor Jonathan Barker of Grace Lutheran Church in Kenosha, Wis., said he felt a thrill seeing the clips of the sermon spread across his social media feeds, popping up from Lutheran colleagues, parishioners and people he would not have guessed to be supportive.

His congregation is part of the Evangelical Lutheran Church in America, and it was active in mobilizing support for Jacob Blake, a Black man shot by a white police officer in Kenosha in 2020. Some 50 years ago, at the height of mainline strength, the Grace congregation had about 1,500 people. Now it has about 40.

“We are willing to do things that other people would find hopeless,” he said. “Our Christian tradition is marked by hope. This feels like a time where we need hope.”

Others who had long ago stopped attending services wondered if there might be a place for them after all.

“It gave me hope — maybe I’ll be able to go to church again,” said Amy Tankard, 59, who lives in a rural part of eastern Virginia.

Ms. Tankard had once been part of a church in the Presbyterian Church (USA), with a female pastor. But the church splintered during the coronavirus pandemic, she said, and the pastor was ousted over a dispute over whether the church should stay closed because of health concerns.

Ms. Tankard told her husband she was not setting foot back in church until the church stopped being so involved in conservative politics.

“It feels like, if you are not with the current government, then you are not with church,” she said. “And I miss it. I think it’s why her sermon meant so much.”

It was too soon to know whether one sensational moment in one pulpit was enough to lead people like Ms. Tankard back to church. Mainline Protestants now comprise about 14 percent of American adults, according to the Pew Research Center. Evangelical Protestants represent about 25 percent, and Catholics 20 percent.

Mr. Trump has abandoned the mainline. He announced in 2020 that he no longer identified with the Presbyterian denomination that confirmed him as a child, but rather as a nondenominational Christian, a tradition closely associated with evangelicalism.

Bishop Budde’s message seemed to be resonating beyond the usual audience for Sunday sermons.

Her most recent book, “How We Learn to Be Brave,” was listed as temporarily out of stock on Amazon Friday afternoon. At that time, the book was No. 4 on the site’s list of best-sellers, 11 spots above Vice President JD Vance’s memoir “Hillbilly Elegy.”

The publisher of Bishop Budde’s book, Avery, an imprint of Penguin Books, was scrambling to reprint “a significant number of books,” said Tracy Behar, Avery’s president and publisher. She declined to share specifics.

At Church of the Transfiguration, Father Shobe pointed out that the sermon was more than just the one brief passage that made headlines. In her remarks, he said, Bishop Budde explored more deeply the concept of unity in complex times.

The last few months had been difficult for many people in the mostly progressive Dallas congregation, he said. But they were determined not to spend the next four years fixating on Mr. Trump’s every attention-grabbing move.

“We’re going to be much more focused on the broader work of the kingdom of God, which is beautiful and good and true,” he said. “If we can focus on that which is beautiful and good and true, we will ride through these four years and find our purpose.”