Christians should be more concerned with principles than individual politicians, says the author of a best-selling book on the perils of Christian nationalism.

Tim Alberta, a journalist and author of “The Kingdom, the Power, and the Glory: American Evangelicals in an Age of Extremism,” says the conflation of faith and political positions can damage the witness of the evangelical church, the faith in which he was raised and practices.

“God is not biting his fingernails over what happens in the United States of America, regardless of who wins or loses some election,” he said in a video interview.

“I think the question is, are the Christian nationalists maligning the reputation of the church? That’s the reputation I worry about the most, because I believe if the church’s reputation is soiled, if the credibility of the gospel of Jesus Christ is diminished, then we as believers have a much harder time carrying that gospel to all the nations and making believers out of an unbelieving world.”

His book has spent four weeks on The New York Times non-fiction bestseller list. In it, Mr. Alberta examines the impact of nationalistic political views on evangelicalism, including the fracturing of the movement during the Trump presidency; the downfall of leaders such as Jerry Falwell Jr., who resigned as Liberty University’s president after a sex scandal; and the rise and influence of others, such as the Rev. Robert Jeffress, senior pastor of the First Baptist Church of Dallas.

The rise of Christian nationalism, which aims to impose Christian ideology on government and society, concerns Mr. Alberta and others who survey the evangelical landscape.

Evangelical churches, including the one his late father pastored, have split in the past five years over Donald Trump’s presidency and its aftermath, including the Jan. 6, 2021, riot at the U.S. Capitol.

After his father’s sudden death in 2019, Mr. Alberta was stunned to find himself attacked by some church members after the funeral. His reporting for The Atlantic on Mr. Trump and his supporters evoked condemnation during intense personal loss.

In his book, his wife says after reading a hostile letter from a church member, “What the hell is wrong with these people?”

What’s wrong is that some evangelicals have traded their countercultural role of meeting persecution with patience and love for attempts at amassing political power, Mr. Alberta said. That’s the opposite of how Christ’s followers and their leaders responded to circumstances in the first century A.D., he said.

“We want to think that in the time of Jesus and in the time of the early church, that it was all flowers and butterflies and unicorns and rainbows, and that everybody was so nice to these Christians, and that’s why Jesus talked about loving your neighbor,” Mr. Alberta said.

“In fact, the culture had never been more hostile than it was to those early believers. They were treated terribly. They were persecuted and oppressed relentlessly for their beliefs,” he said.

Instead of turning to the sword or seeking political power in the Roman system, early believers responded lovingly to those who persecuted them, so much so that ultimately those in power asked what was so special about Christ-followers — and from that question, the small band of believers began to grow, Mr. Alberta said.

“So much of that growth resulted from the church being a truly countercultural movement,” he said. “Not just countercultural because they were a minority, but countercultural because they engaged with their opponents in ways unlike anything the ancient world had ever seen.”

Asked how he would counsel evangelical parents concerned about the introduction of gender ideologies in school with which they disagree, Mr. Alberta made two assertions.

“What I would say to a parent in that situation, where your child is being besieged with what you would view as sort of radical left-wing indoctrination, you can, of course, engage civically to try to change the composition of the school board,” he said. “You can engage as a parent to try and teach them your own biblical values surrounding sexuality and everything else.”

However, he said, parents should also “teach [their] children to understand that these people you may disagree with are made in the image of God and that you, as a Christian, are called to love them. You are called to treat them with dignity, compassion and grace, and if you do that, then everything else will have a pretty funny way of falling into its proper place.”

One evangelical thinker, James Spencer of the D.L. Moody Center, agrees that some Christians can conflate faith and politics.

“As much as we may want to and, in the context of the United States, have the opportunity to influence the government, the political realm is distinct from the Church,” Mr. Spencer said in a statement. “We must take care to distinguish between Church and state because the Church must continue to acknowledge the authority of Christ as part of its unique mission.”

While he’d like to take a break after releasing his book on Christians in American political life, Mr. Alberta said he will likely be drawn in to covering the 2024 presidential campaign. He said he also will keep an eye on the evangelical movement.

“It does feel like we are in a precarious and unstable place as a country, and I have a real concern about the decline of our institutions and just the growing distrust between neighborhoods and communities,” he said. “It feels like we’re approaching a dangerous place in American life.”