Pope Francis has a message for Donald Trump: building a wall on the Mexican border is "not Christian."
The pontiff delivered that message Thursday during a news conference on his flight back to the Vatican from Mexico.
"A person who only thinks about building walls, wherever they may be, and not building bridges, is not Christian," the Buenos Aires-born pontiff said when asked about Trump's now infamous promises to erect a wall aimed at keeping Mexicans out of the U.S.
Francis insisted he was not telling Americans whether or not they should vote for Trump, the Republican presidential candidate who recently lost his lead to rival Sen. Ted Cruz in a new NBC News/Wall Street Journal poll.
"I am not going to get into involved in that," said the pope, who had just wrapped up a tour of Mexico. "I would only say that this man is not Christian if he has said things like that."
Trump responded angrily to the pope's comments, saying in a statement on his website: "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."
He went on to say:
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.
Several other Republican presidential candidates have endorsed building a wall or fence along the U.S.-Mexico border, including Cruz, Sen. Marco Rubio and Ohio Gov. John Kasich. Rubio is Catholic, although he also attends a services at a Southern Baptist megachurch and was a Mormon for three years in his youth.
Francis also weighed in on the Zika virus, which the World Health Organization recently declared a public health emergency. While the Catholic Church is opposed to contraception, Francis said this might be a time when use of it is warranted to prevent the conception of children with a devastating birth defect called microcephaly. He cited Pope Paul VI's decision back in the 1960s to allow nuns to use birth control when women in what was then the Belgian Congo were being raped en masse.
"On the other hand, avoiding pregnancy is not an absolute evil," the Pope said.
But Francis restated the Catholic Church's opposition to abortion in no uncertain terms.
"It is a crime," he said. "It is to kill someone in order to save another. That is what the Mafia does."
Francis, who has already reached out to divorced Catholics and Catholics who have remarried in civil ceremonies, said it is important to bring them back into the fold. But that doesn't mean allowing them to partake in the sacrament of Communion, because divorce is forbidden in the Catholic Church.
"Work towards integration, all doors are open, but we cannot say, 'From here on they can have communion'," he said. "This would be an injury also to marriage, to the couple. It wouldn't make them do the path of integration."
The pontiff addressed reports that his predecessor, Pope John Paul II, had maintained a close friendship with a married woman for 32 years.
The BBC earlier this week aired a documentary about Pope John Paul II's long-term friendship with Polish-born philosopher Anna-Teresa Tymieniecka. Francis said that was no secret and he first heard about it when he was still a cardinal in Argentina.
"I would also say a man who does not know how to have a relationship of friendship with a woman...he's a man who is missing something," said Francis, who stunned conservative Catholics three years ago when admitted he was "dazzled" by a girl he met while training to become a priest.
"A friendship with a woman is not a sin, it's a friendship," he said. "We have not understood the good that a woman do for the life of a priest and of the church in the sense of counsel, help, healthy friendship."
This story first appeared on NBCNews.com.