"A person who thinks only about building walls, wherever they may be, and not of building bridges, is not Christian," the pontiff said on Feb. 18 aboard the papal plane.
"This is not the Gospel. As far as what you said about whether I would advise to vote or not to vote," Francis said. "I am not going to get involved in that. 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."
"If and when the Vatican is attacked by ISIS, which as everyone knows is ISIS' ultimate trophy, I can promise you that the pope would have only wished and prayed that Donald Trump would have been president," Trump said during an appearance in South Carolina that day.
Trump went on to argue that the Mexican government was "using the pope as a pawn" and wrongfully feeding him information.
"That's the Mexican government," he said. "They should be ashamed of themselves for doing so, especially when so many lives are involved and when illegal immigration is so rampant and so dangerous and so bad for the United States.”
Trump then said he "likes" the pope, but considered his comments "disgraceful."
"For a religious leader to question a person’s faith is disgraceful," Trump said in a statement released the same day. “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.”
The two men started softening their public stances in the lead up to this visit. After it was announced that Trump would be paying the Catholic leader a visit, Francis acknowledged that they may not see eye to eye.
When asked about what he expects from his meeting with Trump, given that they have very different views on immigration and climate change, the pontiff replied that he doesn't want to prejudge the president before he has met with him.
"I will tell him what I think, he will tell me what he thinks, but I never wanted to judge someone before I listen to the person first," the pope said aboard the papal plane on May 13.
The pope said he won't try to persuade Trump to embrace his views. "I am not a proselytiser," he said.
When asked if he hopes Trump will soften his stance after their meeting, the pope said it's a "political calculus" he cannot afford.
Instead, Francis said he would be honest and open with Trump and expects to talk about "common things."
"There are always doors that are not completely shut," Francis said. "We need to find the doors that are at least slightly open."
He added that people should say what they think, but "with respect."
For his part, Trump has kept an optimistic tone about the visit.
At an April 20 news conference, Trump said "I look very much forward to meeting the pope."
During his commencement speech at the Coast Guard Academy, Trump said that he will "talk with Pope Francis about the contributions of Christian teachings to the world."
Finally, in a weekly video address previewing his foreign trip, Trump described the meeting as a "great honor."
"I look forward to speaking with the pope about how Christian teachings can help put the world on a path to justice, freedom and peace," he said.