ABOARD THE PAPAL PLANE — Pope Francis said Thursday that Donald Trump is “not Christian” if he intends to build a wall along the U.S.-Mexican border. Trump immediately fired back, saying it is disgraceful for a religious leader to question a person’s faith.

Trump, a leading U.S. Republican presidential candidate, has promised to build a wall along the Mexican border from Texas to California and expel 11 million people who are in the country illegally if elected president. The Pope’s comments en route home from Mexico came hours after he prayed at the Mexico-U.S. border for people who died trying to reach the United States.

“A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian,” Francis said. “This is not in the Gospel.”

Not having heard Trump’s border plans independently, Francis said he’d “give him the benefit of the doubt.” But he added: “I’d just say that this man is not Christian if he said it this way.”

Texas Sen. Ted Cruz, another Republican presidential contender, has also supported building a border wall, and joked that he will make Trump pay for it.

Trump, a Presbyterian, last week criticized Francis’ plans to pray at the border. He said the move was ill-informed and showed Francis to be a political figure being exploited by the Mexican government.

“I don’t think he understands the danger of the open border that we have with Mexico,” Trump said in an interview with Fox News. “I think Mexico got him to do it because they want to keep the border just the way it is. They’re making a fortune, and we’re losing.”

On Thursday, he responded to the Pope’s comments during a campaign stop in Kiawah Island, South Carolina.

“No leader, especially a religious leader, should have the right to question another man’s religion or faith,” he 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.”

He also said the Mexican government has disparaged him to the Pope and separately invoked the Islamic State group, saying that if it attacks the Vatican, “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.”

Asked if he felt he was being used as a pawn of Mexico, Francis said he didn’t know.

“I leave that judgment to you, the people.”

But he seemed quite pleased to hear that Trump had called him a “political” figure, noting that Aristotle had described the human being as a “political animal.”