During a press conference Francis referred to Trump's plans to build a wall between the United States and Mexico if he wins November's election.
"A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," the pontiff said.
"This is not in the gospel," he stressed.
He added: "I say only that this man is not Christian if he said things like that.
"We must see if he said things in that way and in this I give the benefit of the doubt."
Pope Francis made the comments during a press conference at the end of his trip to Mexico.
Donald Trump had already spoken out against Francis's visit.
The pontiff was a "very political person" he said.
"I don't think he understands the danger of the open border we have with Mexico," Trump told Fox News.
