Francis celebrated an opening Mass in St Peter's Basilica with global attention newly focused on the wildfires that are devouring the Amazon rainforest, which scientists say is a crucial bulwark against global warming.
On hand for the service were indigenous people, some with their faces painted and wearing feathered headdresses, as well as more than 180 cardinals, bishops and priests, who donned green vestments like the Pope.
They travelled to Rome from the region for three weeks of debate at a special synod which has become one of the most controversial of Francis's papacy.
Among the most contentious proposals on the agenda is whether married elders could be ordained priests to address the chronic priest shortages in the region. Currently indigenous Catholics in remote parts of the Amazon can go months without seeing a priest or having a proper Mass.
Another proposal calls for the Catholic Church to identify new "official ministries" for women, though organisers have made it clear that priestly ordination is off the table.
Francis's conservative critics, including a handful of cardinals, have called the proposals "heretical" and an invitation to a "pagan" religion that idolises nature rather than God.
They have mounted an opposition campaign, issuing petitions and holding conferences to raise their voices.
In many ways, Francis opened the synod last year, when he travelled to the Peruvian Amazon and demanded that corporations stop their relentless extraction of timber, gas and gold.
Meeting native families in steamy Puerto Maldonado, Francis declared that the Amazon and its indigenous people are the "heart of the church" and demanded that governments recognise their rights to determine the region's future.
The seeds of the Amazon synod, however, long pre-date that visit and even Francis's landmark 2015 encyclical Praise Be, in which he denounced the profit-at-all-cost business interests destroying the rainforest.
The pope, when he was the former Cardinal Jorge Mario Bergoglio of Argentina, drafted the final document of the 2007 meeting of South American bishops in Aparecida, Brazil, which identified the Amazon and its indigenous people as threatened by global economic interests and deserving of the church's utmost attention.
Francis's closest advisers have said the Aparecida document was in many ways a blueprint for the papacy.
Scientists say the vast rainforest's lush vegetation absorbs heat-trapping carbon dioxide from the atmosphere. The moisture given off by its trees also affects rainfall patterns and climate across South America and beyond.
While the number of fires burning in the Amazon declined sharply last month, parts of the rainforest burned at a pace in July and August unseen since 2010.
That fuelled global worries about climate change, put the fires on the agenda of the Group of Seven summit hosted by French President Emmanuel Macron, and directed environmental outrage at the pro-development stance of Brazilian President Jair Bolsonaro.
Mr Bolsonaro has repeatedly said he wants to promote economic development in the Amazon and regularise small-scale illegal mining.
He has also strongly criticised foreign countries for meddling with what he regards as a domestic matter.
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