The battle between Pope Francis and Donald Trump heated up Thursday, with the pontiff questioning Trump’s Christianity and the GOP presidential frontrunner saying it is “disgraceful” for the pope to question his faith.

On the papal plane on the way back to Rome from Mexico, reporters asked Francis about Trump’s plan to build a massive wall along the border.

“Building walls instead of bridges is not Christian; this is not in the Gospel,” the pope said.

The pope said he hadn’t heard about Trump’s plan, but took reporters’ word for it, and said he’d give Trump “the benefit of the doubt.” But he added: “I’d just say that this man is not Christian if he said it this way.”

Trump, a Presbyterian, wasted no time responding.

In a statement, his campaign blamed “the Mexican government” for disparaging Trump to the pope, who he said was being used as a “pawn.”

“For a religious leader to question a person’s faith is disgraceful,” the statement said.

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

The war of words between the two began last week when Trump said Francis’ advocacy on behalf of undocumented immigrants showed that he doesn’t understand the United States’ border issues. He said he suspected that the pope was being manipulated by the Mexican government, and called the pontiff “a very political person.”

“I think he doesn’t understand the problems our country has,” Trump said. “I don’t think he understands the danger of the open border that we have with Mexico. I think Mexico got him to do it because Mexico wants to keep the border just the way it is because they’re making a fortune and we’re losing.”

While in Mexico, Francis prayed at the border for migrants who had died trying to cross into the United States and blessed a group gathered on the US side in El Paso. He has long called on nations to be more welcoming of those escaping persecution and violence in their home countries.

The Vatican responded to Trump Wednesday, calling his remarks “strange,” and basically saying he shouldn’t lecture the pope about immigration. The Vatican spokesman said the pope routinely makes similar comments to European leaders, which “Trump would know if he came to Europe.”

On Thursday, Trump promised to protect Christianity as president.

“I am proud to be a Christian and as president I will not allow Christianity to be attacked and weakened unlike what is happening now with our current president,” the statement said.

Trump’s social media director, Dan Scavino, also lashed out at Francis on Twitter, posting a map of the Vatican and writing:

Scavino’s argument mirrors a similar refrain from a right-wing Italian political party that also points to the Vatican’s walls whenever the pope calls on Europe to be more welcoming to migrants.

The president of Liberty University, Jerry Falwell, Jr., who has endorsed Trump, also slammed Francis.

“Jesus never intended to give instructions to political leaders on how to run a country,” he told CNN Thursday.

GOP candidate Jeb Bush, a Catholic, said he doesn’t question Donald Trump’s Christianity nor anyone else’s, “because I honestly believe that’s a relationship you have with your creator.”

But he did object to the pope’s contention that securing the US border with Mexico by a wall is “not Christian.” He told in Columbia, SC: “I support walls and fencing where it’s appropriate,” along with other forms of border security measures such as drone aircraft monitoring.

As far as Francis’ weighing in on a political debate, Bush repeated what he said in June before the Pope issued his encyclical on climate change: “I think it’s okay to get my guidance as a Catholic from the pope. But certainly not economic policy or environmental policy.”

Another GOP hopeful, Marco Rubio, defended the right of the United States to set its immigration policy, but he refrained from commenting explicitly on what the pope had to say about Trump.

Rubio, a Catholic, said, “Vatican City controls who comes in, when they come in and how they come in as a city state. And as a result, the United States has a right to do that as well.”

“I haven’t seen the context of the Holy Father’s statement, and I won’t comment on it directly,” he said, reiterating that the United States has “not just a right but an obligation to control the process by which people enter” the country.

Trump continued his comments about the pope at a campaign event in Kiawah Island, South Carolina Thursday, saying that “if and when the Vatican is attacked by ISIS, which everybody knows is ISIS’ ultimate trophy, I can promise you the pope would have only wished and prayed that Donald Trump would have been president.”

The crowd applauded.

Trump may be right about the desire of ISIS to attack the Vatican. The terrorist organization’s propaganda magazine put on its October 2014 edition an image of the Vatican flying the ISIS flag and calling for war against the Catholic Church.

During his Mass in Juarez Wednesday, Francis called “forced migration” a “human tragedy,” spurred on by “violence and the hell of drugs.”

“No more death! No more exploitation!” Francis said to hundreds of thousands gathered in Juarez. “There is still time to change, there is still a way out and a chance, time to implore the mercy of God.”

There wasn’t always bad blood between the pope and Trump. About two years ago, Trump tweeted:

After the pope’s Mass in Juarez, both Democratic contenders for the White House took to Twitter to praise Francis.

“We must heed Pope Francis’ call to put compassion at the center of our immigration policy,” Bernie Sanders tweeted Wednesday night.

Hillary Clinton joined in an hour later, tweeting, “Thankful for @Pontifex’s visit to the border and call for immigration reform. We need to keep families together, not break them apart.”