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."
Read more: Pope Francis to Donald Trump: Building Mexico Wall Is 'Not Christian' - NBC News