“For a religious leader to question a person’s faith is disgraceful,” Trump said in a statement that he read out to a crowd at a campaign stop in South Carolina, which holds its primary vote on Saturday.
“I am proud to be a Christian and as president I will not allow Christianity to be consistently attacked and weakened.”
The comments framed an extraordinary public exchange between a candidate and the leader of the Roman Catholic church in the run up to a US presidential election.
Francis issues his remarks after concluding a five-day trip to Mexico, where he delivered a mass before 300,000 people near the US border and decried the “human tragedy” of migrants fleeing violence worldwide.
In a highly symbolic gesture the pontiff climbed a ramp facing the Rio Grande and looked out across the border into US territory, where hundreds of migrants waved at him.
Trump, the brash real estate magnate, has vowed to build a wall on the US southern border to keep migrants from illegally crossing into the United States, a pledge that has caused a firestorm in the presidential campaign where immigration is a hot-button issue.
“Anyone, whoever he is, who only wants to build walls and not bridges is not a Christian,” the pontiff told journalists during his return journey from Mexico, in response to a question about Trump’s anti-immigrant stance.
Trump’s reaction was notable for its tone against a global religious figure.
“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, referring to the Islamic State extremist group controlling wide swathes of Syria and Iraq.
Trump said Mexican authorities 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,” Trump said.
“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.”