Pope Francis arrives in Santa Maria Maggiore basilica carrying a bouquet of flowers upon his return in Rome after his trip to Mexico, Feb. 18, 2016. TIZIANA FABI/AFP/Getty Images

Speaking from his plane Thursday after visiting Mexico, Pope Francis indicated Donald Trump “is not Christian” because of the Republican presidential candidate's proposal to build a wall along the United States’ southern border. Trump, who first suggested the wall in his campaign announcement last year as a way to curb illegal immigration, identifies as Presbyterian.

"A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," Francis said in response to a reporter's question Thursday, according to the New York Times.

Trump shot back almost immediately with the following statement:

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. ISIS would have been eradicated, unlike what is happening now with our all talk, no action politicians.

The Mexican government and its leadership has made many disparaging remarks about me to the Pope, because they want to continue to rip off the United States, both on trade and at the border, and they understand I am totally wise to them. The Pope only heard 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. He doesn’t see how Mexican leadership is outsmarting President Obama and our leadership in every aspect of negotiation.

For a religious leader to question a person’s faith is disgraceful. I am proud to be a Christian, and as president, I will not allow Christianity to be consistently attacked and weakened, unlike what is happening now, with our current president. 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.

Trump adviser Dan Scavino then took to Twitter to respond to the pope's comment:

Thursday wasn't the first time Trump spoke out about the pontiff. ABC News ran a story collecting the billionaire's tweets about the pope last year. Many were flattering messages, including:

But Trump has changed his stance in recent months. The GOP front-runner told Fox Business last week he thought the pope didn't appreciate the issues the U.S. was having with Mexico.

"I think the pope is a very political person, and I think that he doesn’t understand the problems our country has,” he said.