VATICAN CITY (Reuters) - Two giants of Roman Catholicism in the 20th century will become saints on Sunday at an unprecedented twin canonization that has aroused both joy and controversy in the 1.2 billion member Church.

Pope John XXIII, who reigned from 1958 to 1963 and called the modernising Second Vatican Council, and Pope John Paul II, who reigned for nearly 27 years before his death in 2005 and whose trips around the world made him the most visible pope in history, will be declared saints by Pope Francis.

While John died half a century ago, critics say the canonization of John Paul - which sets a record for modern times of only nine years after his death - is too hasty. They also believe he was slow to grasp the seriousness of the sexual abuse crisis that emerged towards the end of his pontificate.

Nevertheless, more than a million people are expected to flock to Rome - many of them to the Vatican - for the ceremony at which Francis will raise two of his predecessors to what the Church calls "the glory of the altars". Large TV screens have been set up around the city to spread out the crowds.

The Church will declare the popes who left an indelible mark on Catholicism led lives of holiness and are worthy of imitation by the faithful. Church investigators have credited them with interceding with God after their death to perform medically inexplicable miracle cures of sick people who prayed to them.

While the late Polish pope is hailed for his role in helping to bring about the fall of communism, critics have questioned his actions as the child abuse scandals - which have since shaken the moral authority of leaders of the world's largest religious denomination - began coming into the open.

Specifically, they have pressed the Vatican over what John Paul knew about sexual abuse by Father Marcial Maciel, the Mexican founder of a disgraced Catholic religious order, the Legionaries of Christ.

Maciel lived a double life for years as a paedophile, womaniser and drug addict while running the rich, conservative order he founded and being held up by the Pope and his aides as an example of an outstanding religious leader.

WARNINGS IGNORED?

John Paul's defenders have said that while aides might have known the allegations were true, they kept much information from him. The pope seemingly ignored the warnings, believing the charges were part of a plot against the Church similar to those by communist authorities in Poland during the Cold War.

Maciel was eventually disciplined in 2006 by John Paul's successor, former Pope Benedict, when the Vatican was forced to admit that decades of allegations were true.

Groups representing victims of sexual abuse have criticised John Paul over the Maciel case as well as his decision to give Cardinal Bernard Law of Boston, who was forced to resign in 2002 after scandals hit the United States, a prestigious job in Rome.

"Popes can't wield massive power yet evade responsibility for massive wrongdoing just because his aides may have carefully shielded him from the gory details," the U.S.-based Survivors Network of those Abused by Priests (SNAP) said in a statement.

But some in the Church think popes should not be made saints, even if they are undoubtedly saintly men. "This is an example of the papacy canonising itself," said Luigi Accattoli, one of Italy's most respected Catholic authors.

Accattoli knew both popes and is sure they were holy men, but has some reservations about the politics of saint-making.

"By canonizing a pope, the papacy confirms itself. It's as if they are saying that the policies of previous popes are untouchable," he told Reuters. "In a sense the Church tries to withdraw itself from judgement by public opinion".

John Thavis, author of the best-selling book "The Vatican Diaries" noted that it was John Paul who wanted more 'ordinary' saints, encouraging the Vatican's saintmakers to find lay people and even a married couple to canonize. "So it's a little ironic that, with his own canonisation, the focus has shifted back to the top of the hierarchy," he told Reuters.

Even the late Cardinal Carlo Maria Martini of Milan, who considered John Paul a holy man, had some reservations about making him a saint. He told a Vatican committee that some of his choices for aides at the end of his life were "not happy ones".

NOT EVEN SAINTS ARE PERFECT

But Father Tom Rosica, the head of Salt and Light Catholic television network in Canada, said canonization was not intended to be a comprehensive evaluation of a papacy. "(Being declared a saint) does not mean that the person was without imperfection, blindness, deafness or sin," he wrote in an essay. "(It means) that a person has lived his or her life with God ..."

Indeed, the overwhelming majority of Catholics are thrilled that John Paul will be made a saint.

Cardinal Karol Wojtyla of Krakow, Poland was elected the first non-Italian pope in 450 years in 1978. He took the papacy on the road, visiting about 140 countries, and for nearly all of them it was their first papal visit.

John Paul was credited with being instrumental in the fall of communism in Eastern Europe in 1989 because of his steadfast defence of the Solidarity trade union in Poland.

He was nearly killed by Turkish gunman Mehmet Ali Agca in St Peter's Square in 1981, and towards the end of his life, his struggle with ill health was seen by millions around the world whenever he appeared on television.

Millions of people attended his funeral in April, 2005, and many cried "Santo Subito" or "Make him a saint immediately".

His successor, former pope Benedict, waived a Church rule that normally requires a five-year waiting period before the preliminaries to sainthood can even begin.

Pope Francis bent the rules for the canonisation of John XXIII, deciding that only one miracle, instead of the customary two, were needed to make him a saint. The rotund John was known as "the good pope," for his kindness, meekness, self-deprecation, and jovial character. Once asked how many people worked in the Vatican, he replied: "About half of them".

John, a compromise candidate in the conclave of 1958 who was not expected to rock the boat, instead started a revolution.

He called the Second Vatican Council, which sat from 1962 to 1965. The first such meeting of the world's Catholic bishops in nearly a century, it changed the face of Catholicism by introducing vernacular to substitute Latin at Masses, encouraged dialogue with other religions and repudiated the 2,000-year-old concept of collective Jewish guilt for the death of Jesus.

(Reporting By Philip Pullella; editing by David Stamp)