After leaving Israel on Tuesday, US President Donald Trump heads to the Vatican on Wednesday to meet Pope Francis, with all eyes on whether they will end their sparring to find common ground.
The two leaders have been at odds over issues from migration to climate change, staking out starkly opposing positions through the media, most notably in 2016 while Trump was campaigning for the US presidency.
But there is a potential for reconciliation: Back in 2013, noting the pontiff's first Christmas as head of the Catholic Church, Trump tweeted that "the new pope is a humble man, very much like me, which probably explains why I like him so much!"
And Francis promised this month that he "won't judge" Trump before hearing him out.
Security will be tight in the world's smallest state, with Swiss Guards joined by anti-terrorism police and sniffer dogs in the wake of the suicide attack at a concert in Manchester, Britain, on Monday claimed by the Islamic State (ISIS) terrorist organization.
During his campaign, Trump evoked the specter of an ISIS attack on the Vatican, saying that "if and when" it happens "the pope would have only wished and prayed that Donald Trump would have been president."
Trump's visit to the Vatican is part of his first trip abroad as president, and follows an initial leg in Saudi Arabia and a visit to Israel.
"No president has ever visited the homelands and holy sites of the Jewish, Christian, and Muslims faiths all on one trip," said US National Security Advisor HR McMaster, describing it as a "truly historic" visit.
"He will bring a message of tolerance and of hope to billions."
On Sunday, Trump rejected the idea of a battle between religions, a sentiment the pope shares.
At their meeting in the papal library at 06:30 GMT, the leaders are expected to discuss issues such as efforts to promote world peace, support for religious freedom and the fight against abortion.
Human rights groups are urging Francis to raise US use of the death penalty and the issues of torture and targeted assassinations with the US president.
Meanwhile the Pope has condemned the idea of using walls to keep out foreigners and slammed those who do as "not Christian."
Trump, who grew up in a Presbyterian family but is not a regular churchgoer, said the accusation was "disgraceful."
And while the pontiff is an ardent supporter of efforts to combat climate change, Trump has pledged to pull the United States out of the COP21 Paris Agreement on reducing greenhouse gas emissions.
The president, who will head to Brussels later Wednesday before returning to Italy for a G7 meeting, did not initially plan to meet the pope.
When the unexpected, last-minute request came, the Vatican squeezed him into an early slot.
Holy See sources said the president would have to enter the papal palace via a side door because the weekly Wednesday mass in Saint Peter's Square takes priority.