The pontiff said “a person who thinks only about building walls… and not of building bridges, is not Christian”.
Donald Trump supports deporting nearly 11 million undocumented immigrants.
Calling himself a “proud Christian”, the Republican presidential hopeful blamed Mexico for the Pope’s remarks, calling them “disgraceful”.
Donald Trump Trump has alleged that Mexico sends “rapists” and criminals to the US.
Pope Francis made the comments at the end of a six-day trip to Mexico.
“A person who thinks only about building walls, wherever they may be, and not of building bridges, is not Christian. This is not the gospel,” he said.
The Pope declined to say whether Americans should vote for Donald Trump, who is leading the Republican race for president.
“I say only that this man is not Christian if he has said things like that. We must see if he said things in that way and I will give him the benefit of the doubt,” he said.
Addressing a rally in South Carolina, Donald Trump responded to Pope Francis’ comments.
“For a religious leader to question a person’s faith is disgraceful. I am proud to be a Christian,” Donald Trump said.
“No leader, especially a religious leader, should have the right to question another man’s religion or faith.”
“[The pope] said negative things about me. Because the Mexican government convinced him that Trump is not a good guy,” Donald Trump added.
He also said the Vatican was ISIS’ “ultimate trophy” and that if it attacked, “the Pope would have only wished and prayed that Donald Trump would have been president because this would not have happened”.
Donald Trump’s Republican rivals, Marco Rubio and Jeb Bush, both Catholics, said they look to Pope Francis for spiritual guidance, not political direction.
Marco Rubio said the US has a right and an obligation to control its borders.
Jeb Bush told reporters he “supports walls where it’s appropriate” and that “Christianity is between he and his creator. I don’t think we need to discuss that”.
Earlier this month, Donald Trump called Pope Francis “a very political person” in an interview with Fox News.
“I don’t think he understands the danger of the open border we have with Mexico,” he said.
American Catholics are seen as an important voting bloc in US elections. Many support Republican candidates because of their opposition to abortion and gay marriage.
Donald Trump has been courting the evangelical Christian vote, often successfully, but his fellow Republican rivals have tried to argue that his religiosity is not sincere.
Ted Cruz’s campaign is now running an advertisement featuring a 1999 television interview Donald Trump gave in which he said he was “very pro-choice” when it comes to abortion.
In January, Donald Trump faced ridicule after flubbing a Bible verse when giving a speech to a Christian university in Virginia.
He has said he is a Presbyterian Christian but has had trouble recalling his favorite Bible verse when asked.
Donald Trump has referred to communion, the Christian sacrament signifying Jesus’ last supper, as having “the little wine” and “the little cracker”.
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