Donald Trump has feuded with other candidates, reporters and TV networks during his run for president.
Now, the front-runner for the Republican nomination is feuding with Pope Francis.
On Thursday, the pontiff criticized Trump for the proposal at the heart of his campaign: a pledge to keep people from crossing into the United States illegally by building a wall on the U.S.-Mexico border.
"I'd just say that this man is not Christian if he said it in this way," Francis told reporters in a midflight press conference after a trip to Mexico.
"A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," Francis said, according to The Associated Press' translation of the press conference.
Trump wasted no time firing back. "If and when the Vatican is attacked by ISIS, which as everyone knows is ISIS's ultimate trophy, I can promise you that the Pope would have only wished and prayed that Donald Trump would have been President because this would not have happened," Trump said in a statement released by his campaign.
"For a religious leader to question a person's faith is disgraceful."
Trump has aggressively courted evangelical Christian leaders and voters during his run for president, though as NPR reported after his visit to Liberty University last month:
"There have been some missteps along the way. Last year, he told an Iowa evangelical gathering he had never asked God for forgiveness — a central tenet of the Christian faith — and he repeated that Sunday on CNN. He's declined to cite his favorite Bible verse or even his favorite testament. And the Presbyterian church he says he attends in Manhattan has said he's not an active member."
Trump still won the endorsement of the school's president, Jerry Falwell Jr., after that appearance.
For his part, Francis has not shied away from politics since becoming pope. While simply repeating long-held church positions on the dangers of capitalism and globalization, concern for the environment, and other social justice issues, he has repeatedly made headlines over the course of his career as pontiff and, before that, cardinal, for challenging political leaders.
The first Latin American pope has made immigration a top concern. Earlier this week, he celebrated Mass along the U.S.-Mexico border, saying immigrants crossing the border "are 'cannon fodder,' persecuted and threatened when they try to flee the spiral of violence and the hell of drugs."
Francis even floated the idea of drawing attention to immigration concerns by entering the U.S. via the Mexican border when he made his first visit to the country last fall.
ROBERT SIEGEL, HOST:
And as I said earlier, Pope Francis was also asked about presidential candidate Donald Trump - specifically, about Trump's plan to build a border wall between the U.S. and Mexico. The pope was critical of that idea and his answer implied that Trump, quote, "is not Christian," unquote. Trump himself wasted no time condemning the pope for those comments. And NPR's Scott Detrow joins us to sort this all out. Hi, Scott.
SCOTT DETROW, BYLINE: Hey, Robert.
SIEGEL: What exactly did the pope say?
DETROW: So here's the full translated quote. "A person who thinks only about building walls wherever they may be and not building bridges is not Christian. This is not in the gospel." Now, Pope Francis did not mention Donald Trump by name. And he said he's not going to get involved in telling Americans how to vote. But it's clear from the context and the question that was posed to Pope Francis, this was all about Donald Trump.
SIEGEL: And tell us what Donald Trump had to say about that.
DETROW: He was pretty forceful. Within an hour of the pope's comments going out across the Internet and taking over Twitter it seemed, Donald Trump blasted Pope Francis. He put out a statement which he read during a South Carolina campaign stop.
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DONALD TRUMP: If and when the Vatican is attacked by ISIS, which as everyone knows, is ISIS's ultimate trophy, I can promise you that the pope will have only wished and prayed that Donald Trump would have been president.
DETROW: Now, Trump also said that Mexico basically put Francis up to criticizing him. Trump has been working hard to court Evangelical Christian voters in early primary states like South Carolina. Of course, that's a different end of the Christian-political spectrum, especially given how much liberal Catholics have really been drawn to Pope Francis. But Trump has never backed down when people criticize him. And in fact, we've seen him almost trolling the Vatican this afternoon on Twitter, retweeting pictures of the wall that does surround Vatican City.
SIEGEL: Immigration is a core issue for Trump and his supporters. It's also a priority for the pope, so it's really not all that surprising that they clashed over it.
DETROW: That's right, this is something both of them feel very strongly about. The issue is at the heart of Donald Trump's campaign to a point that it's almost a call and response at Trump rallies. Trump will ask the crowd who's going to pay for the wall, and the crowd responds Mexico. So this is something Trump's been talking about since day one. But Pope Francis has also spent a lot of time talking about immigration both here in the United States and also in Europe when it comes to the Syrian refugee crisis. Pope Francis, of course the first Latin American pontiff, he was coming back from a visit to Mexico where he made a point of highlighting immigration. In fact, earlier this week, Pope Francis visited the U.S.-Mexico border right across from El Paso and celebrated Mass there. And he called immigration a humanitarian crisis and said that immigrants crossing into the U.S. are persecuted and threatened. So again, this is a personal issue for the pope. He understands symbolism, he understands the political implications when he gets into things like this. And in fact, he had publicly thought about - when he came to the U.S. in September, he told reporters that he had thought about entering the U.S. through the Mexican border in order to make a point about immigration.
SIEGEL: Donald Trump who has made a lot of enemies, I think one could say, during this campaign, or at least insulted quite a few people. The head of the Roman Catholic Church would seem to be the highest-ranking target of his (unintelligible).
DETROW: He's feuded with other candidates, he's feuded with reporters, with TV networks and now Pope Francis.
SIEGEL: OK that's NPR's Scott Detrow. Thanks, Scott.
DETROW: Anytime. Transcript provided by NPR, Copyright NPR.