ABOARD THE PAPAL PLANE — Thrusting himself into the heated American presidential campaign, Pope Francis declared Thursday that Donald Trump is “not Christian” if he wants to address illegal immigration only by building a wall along the U.S.-Mexico border.

Trump fired back ferociously, saying it was “disgraceful” for a religious leader to question a person’s faith.

The rare back-and-forth between pontiff and presidential candidate was the latest astonishing development in an American election already roiled by Trump’s free-wheeling rhetoric and controversial policy proposals, particularly on immigration. It also underscored the popular pope’s willingness to needle U.S. politicians on hot-button issues.

Francis’ comments came hours after he concluded a visit to Mexico, where he prayed at the border for people who died trying to reach the U.S. While speaking to reporters on the papal plane, he was asked what he thought of Trump’s campaign pledge to build a wall along the entire length of the border and expel millions of people in the U.S. illegally.

“A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian,” he said. While Francis said he would “give the benefit of the doubt” because he had not heard Trump’s border plans independently, he added, “I say only that this man is not a Christian if he has said things like that.”

Trump, a Presbyterian and the front-runner for the Republican presidential nomination, responded within minutes.

“For a religious leader to question a person’s faith is disgraceful,” he said at a campaign stop in South Carolina, which holds a key primary on Saturday. “I am proud to be a Christian, and as president I will not allow Christianity to be consistently attacked and weakened.”

Trump also raised the prospect of the Islamic State extremist group attacking the Vatican, saying that if that happened, “the pope would have only wished and prayed that Donald Trump would have been president because this would not have happened.”