When Italians say two people are "like devil and holy water," it means they are poles apart, and the saying seems to apply perfectly to Pope Francis and US President Donald Trump, who will meet for the first time at the Vatican on May 24.
Last week, a life-sized mural appeared on a Roman wall that appeared to capture the incompatibility: it showed the pontiff kissing a devil-like Trump with red horns and featured the inscription "The Good forgives the Evil."
"It''s not a personal issue between them, they just have different cultural, intellectual and moral outlooks," Massimo Faggioli, professor of Theology and Religious Studies at Villanova University, in the US, told dpa.
In their most famous exchange to date, Francis last year dismissed Trump's campaign promise to build a wall on the US border with Mexico as "not Christian", a statement that the then Republican candidate called "disgraceful".
The pope is a one-time slum priest from Argentina who is suspicious of capitalism and wants "a poor church, for the church"; the US leader is a brash real estate billionaire and reality TV star who loves to boast about his wealth.
In office, Francis has tirelessly campaigned for the rights of migrants and prison inmates, flagged up climate change as an emergency, refused to link Islam to terrorism, condemned greed and selfishness and urged a stop to the global arms race.
On his part, Trump has accused Mexicans of being "rapists", issued bans against Muslims entering the country, dismissed climate change as a hoax and promised "one of the greatest military buildups in American history.
"In the past, US administrations have often been at odds with the Vatican on a range of issues, including the invasion of Iraq, the use of condoms against the spread of AIDS, the death penalty, abortion and gay marriage.
But the gap between Francis and Trump "is the widest I can remember," Faggioli said, arguing that the pontiff's relatively liberal views are also disconcerting for the conservative leadership of the US Catholic Church.
"Many American cardinals feel disappointed, and somewhat betrayed by a pope who says things about homosexuals, family life and divorce which are very far from what they see as the priority," the professor argued.
Even before Trump came to power, Francis' relationship with the US was problematic. Hailing from a region that normally associates the US with colonial arrogance, he has been cast as a Marxist by American right-wingers.
However, there is potential for cooperation between two men who share a penchant for making disconcertingly candid remarks and who like to rattle their respective establishments, albeit in very different ways.
"Both Trump and Francis are populists, seeing their legitimacy as coming from the people rather than elites, and both take a certain pride in the fact that elites tend to view them with alarm," Vatican expert John L Allen Jr wrote on the Crux website.
The defence of the Christian minority in the Middle East and the need to end the bloodshed in Venezuela are just some of the areas on which Trump could see eye to eye with Francis, a spiritual leader who also commands one of the world's largest diplomatic networks.
"Francis and Trump likely recognise that the United States is the world's most important hard power and the Vatican its leading soft power, so it's in everyone's interests for these two players to be on good speaking terms," Allen said.