It is sometimes described as the "Olympics of religion".
Some prefer to call it a Catholic Woodstock - a festival of peace, love and Christianity.
Almost a quarter of a century after it came into being, World Youth Day - which is actually a six-day event that begins on Tuesday - continues to exert a magnetic pull on young Catholic pilgrims from all over the world.
The flags draped over their shoulders not only provide a blaze of national colour but show how far they have travelled to get to this far-flung corner of the planet.
Why, then, have over 100,000 international pilgrims made the journey to see the 81-year-old Pontiff, Pope Benedict XVI?
Cloaked in the saltire, the flag of Scotland, Christopher Kinghorn, explained why.
"I think the event is excellent because it unites the different cultures from around the world who share the same faith and it brings us closer together," he said.
At a time when many of the church's teachings on sexuality and birth control are criticised for being out of sync with a rapidly changing world, Chris Daniels from Atlanta, Georgia, believes that young Catholics should restate their articles of faith.
"I understand most of the Catholic Church teachings are anti-cultural now," said Chris.
"But we're looking to change the culture, to make it more pro-life, pro-Catholicism, pro-Jesus."
The aim of this religious festival, which is held every two or three years, is to energise young pilgrims in a manner which not only strengthens their faith, but safeguards the future of the Catholic Church.
But the message of World Youth Day has been undercut by the controversies of the recent past: the sexual abuse scandal which refuses to go away.
In the lead up to the event, Australia's most senior Catholic, Cardinal George Pell, has faced fresh allegations of a cover-up in the case of a young man indecently assaulted in the early 1980s.
In a report which generated a barrage of negative headlines, the ABC's Lateline programme revealed the shocking story of Anthony Jones, a religious education coordinator, who was indecently assaulted by a Sydney priest, Father Terence Goodall.
Originally, the Cardinal dismissed Anthony Jones's allegations of sexual assault, saying the church had received no other complaints against Father Goodall.
But newly-uncovered documents reveal that Cardinal Pell was aware of another allegation against the priest.
"It destroyed my faith, ripped it to pieces," Anthony Jones told Lateline.
"I now hate Catholicism because of what Cardinal Pell has done to me. More so than what Father Goodall did to me."
In what looked like an exercise in damage limitation, Cardinal Pell has set up a consultative panel headed by a retired New South Wales Supreme Court judge.
Pope Benedict, meanwhile, has arrived vowing to apologise to victims of sexual abuse, as he did during his recent visit to America.
But will that be enough to placate the victims?
"It is a very arrogant church that behaves like this," says Chris MacIsaac of the victims' support group, Broken Rites.
"There's been very little compensation and no compassion. This has to be a meaningful apology and action has to follow."
Ideally, she wants Pope Benedict to personally apologise to Anthony Jones.
To guard against the Pope's visit being disrupted by demonstrators, the New South Wales government brought in restrictive new laws which threatened heavy fines for "behaviour that causes annoyance or inconvenience to pilgrims attending the World Youth Day event".
The laws were denounced by protesters planning to hand out condoms or wear anti-Catholic T-shirts as "backward" and "appalling".
The New South Wales Bar Association agreed, calling the laws "unnecessary" and "repugnant" - and they were later overturned by a court in Sydney.
The Pope will make a grand, ceremonial entrance into the heart of Sydney on Thursday, with a papal boat-a-cade around the harbour.
World Youth Day climaxes with a papal mass at Randwick racecourse on Sunday.
This has been an intricately choreographed event, and its staging benefits greatly from the beautiful backdrops that Sydney offers.
This should be the most telegenic World Youth Day ever mounted. But the headlines have so far been dominated by the more ugly side of the Catholic Church.