By Wiktor Szary and Tadeusz Kolasinski

WARSAW (Reuters) - When Polish bishops issued a letter inviting Catholic youths to attend an international gathering to be held in Poland this week, the name of its host, Pope Francis, was conspicuously absent.

Instead, the bishops spoke about John Paul II, the late Polish-born pontiff who holds an iconic status in Poland for his role in inspiring the nation to stand up to communist rule. John Paul, who died in 2005, was made a saint in 2014.

While the bishops say Francis - and not the ghost of his predecessor - will be the centerpiece of World Youth Day events, the omission reflects a sense of disquiet among senior clergy about his calls for a more inclusive and merciful Church, a message that contrasts with the preaching in many Polish churches.

Poland remains one of Europe's most Catholic and conservative nations, with about 90 percent of citizens declaring allegiance to the Church. Its government openly calls for Christian values to be present in daily life and politics.

"I am absolutely convinced the meeting between Francis and the Polish Church will be challenging for both sides," said Jaroslaw Makowski, a liberal Polish theologian.

"When he was invited to Poland in 2013, he was unknown ... But after a few months it became clear he isn't what was expected ... but someone who wants to shake up the Church and push it off the path familiar to the Polish Church."

The Vatican expects hundreds of thousands of young people from all continents to turn out to see the pope during his five-day visit that starts on Wednesday in the southern city of Krakow, where John Paul was archbishop before he became pope in 1978.

Privately, Vatican officials said they expect Francis to wow young people in Poland just like he did at the last World Youth Day in Brazil three years ago.

However, although direct criticism of the Argentine-born pontiff is rarely heard, Polish Roman Catholic bishops publicly disagree with his views on issues such as homosexuals and divorce, in an echo of conservative concerns elsewhere that he threatens to dilute Church teachings.

A conservative Polish commentator, Tomasz Terlikowski, said the popularity of John Paul in Poland was driven, at least in part, by the late pontiff's adherence to doctrine.

"The World Youth Day, like any previous meeting between John Paul II and young people, serves as ... proof the Church in the West is losing them ... because of what it offered them," Terlikowski wrote in DoRzeczy weekly this week.

"The young have been leaving the Church because what it offers them is equal to that of (the secular) world, only veiled in incense," he wrote.

"Will Francis be able to inspire the youth the way John Paul did? It will depend on what he requires of them," he said.

The World Youth Day events are an effort by the Vatican to inspire Catholics at a time when rival denominations, secularism and sexual and financial scandals continue to lead some to abandon the Church.

ENCROACHING MODERNITY

Poland has undergone a significant shift in its political landscape since the Euroskeptic, conservative Law and Justice (PiS) party in October ended nearly a decade of secular-minded government.

Its leaders tapped Poland's brand of patriotism infused with Catholic piety to build up popularity among voters and ensure the backing of the clergy, influential across provincial Poland.

"Poland is an oasis of freedom in the world, where values that form our identity are being destroyed," Prime Minister Beata Szydlo said during a religious ceremony this month.

"We have a foundation in Poland, where the words 'God, honor and homeland' have guided us for centuries."

Despite being rooted in Christian values, her government disagrees with Francis on issues such as refugees and the environment. It opposes mandatory European Union quotas for accepting migrants and promotes as coal as an energy source.

Church leaders, in turn, have sought the support of conservative politicians to enshrine their view of society in law, liberal observers say, mindful that encroaching modernity is shrinking the ranks of churchgoers.

Since PiS came into power, Catholic officials have pushed for a further tightening of Poland's already restrictive abortion laws, for example.

"The Church has forged an alliance with the government because, using law, it can exert more control," said Makowski.

The Polish Catholic Church is nonetheless divided and many secular theologians, priests and churchgoers would welcome Francis' message that the Church should be more compassionate toward "imperfect" Catholics, such as remarried divorcees.

Divorce rates in Poland are close European averages, and regular mass attendance has fallen by almost 10 percentage points in the last decade to about 40 percent.

More than 70 percent want access to contraception, even though Warsaw Archbishop Henryk Hoser has said using it introduced an "element of disease" into a woman's body.

Nearly 80 percent of Poles say in-vitro fertilization should be accessible for infertile couples, whereas bishops have said lawmakers who voted for a law to regulate IVF should be denied communion.

Polish bishops have made it clear any talk of watering down church doctrine was unacceptable.

"All should avoid a divorce mentality. Every separation of spouses offends God; moreover, it causes much harm," they said in a document published ahead of last year's gathering of bishops in the Vatican to discuss the issue.

The Polish Bishops' Conference spokesman, Father Paweł Rytel-Andrianik, said the letter inviting young Poles to this week's gathering in Poland was one of many documents issued about the event.

"The pope is an intrinsic part of the World Youth Day," he said.

(Additional reporting by Justyna Pawlak, Marcin Goettig and Philip Pullella; Writing by Justyna Pawlak; Editing by Alison Williams)