Pope Francis will welcome President Trump to the Vatican on May 24, as part of Trump's first foreign trip as president.
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's flight back from Portugal today.
The pope said he won't try to persuade Trump to his views: "I am not a proselytiser," the pope 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.
Why Trump chose Saudi Arabia for first foreign trip Donald Trump: What he said about the pope Pope Francis says Donald Trump is 'not Christian'Instead, the Catholic leader 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. We need to find the doors that are at least slightly open," Francis said.
The pope added that people should say what they think, but "with respect."
The two leaders differ greatly on immigration and climate change. Trump has said he wants to withdraw from the Paris climate agreement and has declared climate change to be a "hoax." Pope Francis has written an encyclical on the environment, in which he calls for global action to combat climate change.
As a candidate, Trump promised to build a massive wall on the U.S.-Mexico border and crack down on illegal immigration. Francis has said the "protection of migrants" is a "moral duty."
Reaching the end of his trip to Mexico in February 2016, Francis was asked about Trump in an interview. "Can a good Catholic vote for this man?" one reporter asked the pope.
The pope replied that "a person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian."
That same day, Trump responded to the pope's comments during a campaign rally in South Carolina, telling supporters he is "a good Christian." He also accused Mexico of using the pope as a "pawn."
"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.
Trump, who is a Presbyterian, then said for a religious leader to question a person’s faith is "disgraceful."
On his foreign trip beginning late next week, Trump also plans to travel to Saudi Arabia and Israel, and to attend the NATO meeting in Brussels.
ABC News’ Phoebe Natanson and Meghan Keneally contributed to this report.