The two men's worldviews collided head-on early last year, when Francis was sharply critical of Trump's election campaign pledge to build an impenetrable wall on the Mexican border and his declaration that the United States should turn away Muslim immigrants and refugees.

"A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," Francis said at the time . The pontiff has been a vocal advocate for immigrants and aiding refugees, particularly those fleeing the violence in Syria, deeming it both a "moral imperative" and "Christian duty" to help.

Trump hit back by calling Francis's comments "disgraceful" for doubting his faith.

But despite their previous disagreements, Al Jazeera's James Beys, reporting from Rome, said that many expect a "pretty standard meeting" between a head of state and the pontiff.

On Wednesday, Trump will be given a tour of the Vatican after he arrives and will then first meet with the pope in his library. The two men will then be left alone with a translator to hold a private discussion before emerging again to exchange gifts and farewells.

Trump's visit to the Vatican has been met with a number of smaller protests. Climate change activists projected the words "Planet Earth First" on the massive dome of St. Peter's Basilica at the Vatican on Tuesday night as Trump arrived in Rome.

Protests are also expected on Wednesday in Rome and later in the week when Trump travels to Brussels for a NATO meeting and Sicily for a G7 gathering.

The visit to the Vatican is part of Trump's first foreign trip as US president, and comes after stops in Saudi Arabia, Israel and the occupied Palestinian territories.

News24