Pope Francis has questioned Donald Trump's Christian faith over his vow to build a border wall to keep out immigrants, sparking a backlash from the Republican presidential candidate. "Anyone, whoever he is, who only wants to build walls and not bridges is not a Christian," the pontiff told journalists Thursday during his return journey from a trip to Mexico. He was responding to a question about the billionaire's anti-immigrant stance. "Vote, don't vote, I won't meddle. But I simply say, if he says these things, this man is not a Christian," Francis said. "We need to see if he really said them and for this I will give him the benefit of the doubt." After initially launching into an audacious denunciation of Francis, Trump later softened his tone dramatically. "For a religious leader to question a person's faith is disgraceful," Trump said in a statement delivered on a campaign stop in South Carolina, which holds its Republican primary this weekend. But hours later, at a townhall event, the real estate tycoon heaped praise on the pontiff. "Oh, the pope is a wonderful guy," Trump said, suggesting the media had hyped and exaggerated the tone of his earlier statement.

Trump said Mexican authorities had told the pope only "one side of the story."

"He didn't see the crime, the drug trafficking and the negative economic impact the current policies have on the United States.

"They are using the pope as a pawn and they should be ashamed of themselves for doing so, especially when so many lives are involved and when illegal immigration is so rampant."

It was not the first time Trump had issued the pawn jibe, and Francis was asked about it on the plane.

"Am I a pawn of the Mexican government? I leave that to your judgement, to the people to judge," he said.

The 79-year-old Argentine, a fervent critic of the freewheeling capitalism espoused by Trump, also said he was proud to have been branded a politicized pontiff by the tycoon.

"Thanks be to God if that is what he said, because Aristotle defined man as a political animal: at least (it means) I am a human person," the pope said.

Trump has promised to end illegal immigration by building a wall along the Mexican border, which stretches more than 3,100 kilometers (1,950 miles) -- a third of it currently covered by high-security fencing.

One of Trump's Republican rivals, former Florida governor Jeb Bush, was charitable towards him in remarks addressing the contretemps.

"I don't question anybody's Christianity, because I honestly believe that's a relationship you have with your creator," Bush, a devout Catholic, told reporters.

Speaking of building a wall and other forms of border control, he added: "That's not an un-Christian thing to do, to make sure people don't come across our border illegally. That's a just thing to do."