Pope Francis stated this week that Donald Trump's proposal of building a wall along the US-Mexico border to address illegal immigration is "not Christian."

Trump fired back saying that the pope would wish he were president if the Islamic State attacked the Vatican.

"If and when the Vatican is attacked by the 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," he said.

The pope was asked about his thoughts on Trump's proposal to keep illegal immigrants out if he becomes president.

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

Trump added that it was "disgraceful" of Francis to question his beliefs. He adds that he is a Christian and, if elected, he "will not allow Christianity to be consistently attacked and weakened."

When asked if American Catholics should vote for Trump, the pope did not want to get involved. He reiterates that it is un-Christian of the presidential candidate to release such statements.

Their exchange continued with Trump saying the pope is a "political person." He adds that he not knowledgeable of the problems of the country and that there are dangers with the open borders the U.S. have with Mexico.

Pope Francis responded by saying: "Thank God he said I was a politician because Aristotle defined the human person as 'animal politicus'. So at least I am a human person."

Trump accused the Mexicans of turning the pope against him because their government allegedly wants to "continually rip off" the U.S. He was dismissive of the pope's remarks and does not take it personally.

Francis was on his way to Rome after a six-day visit to Mexico. The statement came up during the news conference to which he also addressed the Roman Catholic Church's stance on abortion and birth control in the wake of the Zika virus outbreak in Latin America.

The pope made his thoughts clear by saying abortion and birth control are different. He condones abortion, saying it is "a crime, an absolute evil" but considers avoiding pregnancy via contraception less evil. He was open to allow exception for contraception in cases like these and called for doctors to do their best to produce vaccines against the virus.