Jay Parini , a poet and novelist, teaches at Middlebury College . His most recent novel is "The Damascus Road," a novel about St. Paul. The views expressed in this commentary belong to the author. View more opinions at CNN.

(CNN) The dam has broken: Some evangelical Christians are turning against Donald Trump in the immediate wake of his impeachment by the House of Representatives. This was bound to happen, as Trump is the least likely man in the known universe to support if you wish to take into account the example and teachings of Jesus. But I didn't expect the reversal to come with such thunder, from Christianity Today, a flagship publication of the evangelical movement.

This magazine represents the movement in a profound way. It was founded by Billy Graham himself in 1956, and it's been a beacon for evangelicals. My father, who was a Baptist minister, waited eagerly for its arrival in the mail, and I did as well. Over the years, I've admired its consistently level-headed approach to Christianity as a faith that must bear upon the actions of the individual.

Mark Galli, its current editor (who is leaving the publication in two weeks) takes on Trump directly -- a courageous move on his part, as his magazine has largely been apolitical. "The facts in this instance are unambiguous: the president of the United States attempted to use his political power to coerce a foreign leader to harass and discredit one of the president's political opponents," Galli writes. He draws the obvious conclusion for Christians: "That is not only a violation of the Constitution; more importantly, it is profoundly immoral."

Galli goes further, digging into the behavior of the man in the Oval Office, noting that Trump "has dumbed down the idea of morality in his administration." He gets specific: "He has hired and fired a number of people who are now convicted criminals." As if that wasn't enough, Galli adds, "He himself has admitted to immoral actions in business and his relationship with women, about which he remains proud. His Twitter feed alone -- with its habitual string of mischaracterizations, lies, and slanders -- is a near perfect example of a human being who is morally lost and confused."

Galli's warning to Christians is clear. "To the many evangelicals who continue to support Mr. Trump in spite of his blackened moral record, we might say this: remember who you are and whom you serve," Galli writes. "Consider how your justification of Mr. Trump influences your witness to your Lord and Savior. Consider what an unbelieving world will say if you continue to brush off Mr. Trump's immoral words and behavior in the cause of political expediency. If we don't reverse course now, will anyone take anything we say about justice and righteousness with any seriousness for decades to come?"

Galli also acknowledged Friday in an interview on CNN's "New Day" that his stand is unlikely to shake loose Trump's strong hold on this voter segment, a crucial portion of his political base. Galli's move is even more admirable when you consider that he published his editorial even knowing that, as he said in his interview, he's not optimistic that his editorial will alter Trump's support among white evangelicals.

It's not a stretch to say that white evangelicals put Trump into office in 2016. About 80% of them voted for him. They did so because of the abortion issue, mostly. They wanted pro-life judges throughout the justice system. But this was a devil's bargain, at best.

Younger evangelicals, those under 45, have been slowly but steadily moving away from Trump during the past two years or so, unhappy about his example.

A key topic that has driven them away is immigration. Loving your neighbor as yourself has always been a bedrock Christian value. And Trump's stance on immigrants (especially those of color) has upset the younger generation of evangelicals, with two-thirds of them saying in surveys that immigrants strengthen our country, bringing their work ethic and talents with them from Mexico or Central America or Syria.

Climate change is another issue that has caught the imagination of younger evangelicals. "I can't love my neighbor if I'm not protecting the earth that sustains them and defending their rights to clean water, clean air, and a stable climate," Kyle Meyaard-Schaap, a national organizer for Young Evangelicals for Climate Action, told Grist . Needless to say, Trump's contempt on this subject grates badly on these young Christians.

Perhaps naively, Americans have always looked to the presidency for exemplary moral behavior, and when there are obvious personal or moral failures, as with Nixon and Clinton, there is disappointment, even anger. But if you're a Christian -- and I lay claim to this for myself -- you understand that it's human to fail at perfect behavior. There is always forgiveness. And, as T.S. Eliot wrote, "Humility is endless."

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Humility lies at the heart of Christian behavior. As does honesty.

In these, Trump has set a terrible example, and he's now been taken down for this by an important Christian voice. If only another 10 percent of evangelicals take this seriously, and I suspect they will, Donald J. Trump's presidency is destined for the ash heap of history.