"I think it will be a moment of truth. The differences are real," says one expert - particularly on climate change and migration
(Corrects year in paragraph two to 2013)
* Trump and Pope Francis will hold first meeting on Wednesday
* Two differ on issues like climate change and migration
* Pope says will be "sincere" with Trump, seeks "doors not closed"
* Trump changed mind after first excluding Vatican on Europe visit
By Philip Pullella
VATICAN CITY, May 22 (Reuters) - Several days after Pope Francis was elected four years ago, he was criticised by Donald Trump for returning to the Church-run residence he was staying at in Rome to pay his bill in person.
"I don't like seeing the Pope standing at the checkout counter of a hotel in order to pay his bill. It's not Pope-like!" Trump tweeted on March 19, 2013.
On Wednesday, Trump will see for himself if the pontiff is papal enough when he walks down the frescoed halls of the Apostolic Palace. The venue of the meeting with Francis - who shunned the palace and lives in a Vatican guest house - will be the study he uses for official occasions.
Their first meeting comes after Trump sparred with the pope on issues such as migration, climate change and Islam.
"I think it will be a moment of truth. The differences are real and they have not been invented by the media, as some Trump supporters have claimed," said Massimo Faggioli, professor of theology and religious studies at Villanova University.
Last year, in response to a question about then-candidate Trump's views on immigration and his intention to build a wall along the U.S. border with Mexico, Francis said a man who thinks about building walls and not bridges is "not Christian".
Trump, who grew up in a Presbyterian family but does not attend church regularly, shot back that it was "disgraceful" for the pope to question his faith.
"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 during the campaign.
After Trump was elected, his barbs ended, and people close to Francis say he does not hold grudges and wants to turn the page.
"You have to distinguish between Trump the candidate and Trump the president," said Father Antonio Spadaro, a close associate of the pope's and editor of the prestigious Jesuit journal Civilita Cattolica.
"Francis does not divide the world between the good and the bad, he does not have this Hollywood-esque view of the world," Spadaro said in an interview.
NO PLANS FOR A FIGHT
"I think it will be a frank and realistic meeting, not one revolving around preconceived concepts. It is not a meeting that is being held in order to fight. It is a desire to meet despite all of the things that preceded it," Spadaro said.
Earlier this month Francis told reporters he would be "sincere" with Trump despite their differences.
"There are always doors that are not closed. We need to find the doors that are at least partly open, go in, and talk about things we have in common and go forward, step by step," he said.
One door that the Vatican hopes is still open is on climate change.
In March, Trump signed an executive order dismantling environmental regulations enacted under his predecessor Barack Obama, but the Vatican is hoping Washington will not abandon the international Paris Agreement in 2015 aimed at shifting the global economy from fossil fuels.
Francis has made defence of the environment an important plank of his papacy, backing scientific opinion that global warming is caused mostly by human activity.
Vatican sources said the pope's inner circle was heartened that Trump had toned down his previously harsh rhetoric about Muslims during his visit to Saudi Arabia at the weekend.
As a candidate, Trump proposed temporarily banning Muslims from entering the United States. In office, he ordered temporary bans on people from several Muslim-majority countries, which were blocked by courts that ruled they were discriminatory.
Trump at first did not plan to stop in Rome during his visit to Europe, which some in the Vatican saw as a snub. When he changed his mind, the Vatican squeezed him in at 8:30 a.m. on a Wednesday morning, an unusual day and an unusually early time.
Francis holds his weekly audience with the general public on Wednesday at 10 a.m. in St Peter's Square. Vatican sources say it is significant that the pope did not cancel or delay it.
"His commitment to the people comes first," one senior source said, noting that because the square will be filled with people, Trump is expected to enter the Vatican though a tiny back gate used by employees instead of the usual entry used by heads of state.
"It will not be the solemn, triumphal entrance he may have wanted," he said. (Reporting by Philip Pullella; Editing by Sonya Hepinstall)
Our Standards: The Thomson Reuters Trust Principles.