No stranger to controversy, Trump, the longtime party front-runner in national opinion polls, has vowed if elected president to build a wall between the United States and Mexico to
keep out immigrants who enter illegally.
In a freewheeling conversation on his flight home from a visit to Mexico on Thursday, Francis told reporters, "A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian."
The pope said he did not want to advise American Catholics on whether or not to vote for Trump, and it was not immediately clear what impact his remarks would have on Republicans likely to vote in the November 8 election.
Republican Catholics appear to support Trump more than other Republicans do, according to a Reuters/Ipsos opinion poll.
"I say only that this man is not Christian if he has said things like that."
As of 2014, 71 per cent of the US population identified as Christian, 21 per cent of them Catholic, according to the Pew Research Center.
Asked if American Catholics should vote for someone with Trump's views, Francis said, "I am not going to get involved in that.
"I say only that this man is not Christian if he has said things like that. We must see if he said things in that way and in this I give the benefit of the doubt," he said.
Trump, a real estate developer and former reality TV star, said, "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," Trump said in a speech in Kiawah Island, South Carolina, using an acronym for the Islamic State militant group.
"For a religious leader to question a person's faith is disgraceful," he said.
"I am a very nice person. And I'm a very good Christian."
Trump has said his book The Art of The Deal is his second favourite book after the Bible.
"I am a very nice person. And I'm a very good Christian. Because the pope said something to the effect that maybe Donald Trump isn't Christian, ok? And he's questioning my faith. I was very surprised to see it, but I am a Christian. I'm proud of it."
Trump has said he would deport millions of illegal immigrants if he wins the White House.
Last week, responding to the pope's plan to visit the US-Mexican border, he said that Pope Francis did not understand the issues involved.
"The pope is a very political person. I think he doesn't understand the problems our country has. I don't think he understands the danger of the open border that we have with Mexico," Trump told the Fox Business Network.
Asked about being called a "political person", Francis said on Thursday, "Thank God he said I was a politician because Aristotle defined the human person as 'animal politicus.' So at least I am a human person."
