"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.
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."
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."
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.