The general assembly of the Synod of Bishops gathers in the wake of an unprecedented two-year global consultation that will also address topics such as women deacons and priestly celibacy. It will take place over four weeks in Rome.
The pope stressed welcome in his opening address, seeking to make the Church of 1.3 billion faithful more tolerant amid changing social norms and demographics, including the growth of Christianity outside the West.
"The Church is here for you! The Church of open doors is for everyone, everyone, everyone," said the 86-year-old at a large mass in St Peter's Square.
"In such a complex time as ours, new cultural and pastoral challenges emerge that call for a warm and kindly inner attitude so that we can encounter each other without fear," he said.
Warning against "a rigid Church," the Argentine pontiff said he hoped the forum would avoid "human strategies, political calculations or ideological battles."
However, he said the intention was not to carry out "a parliamentary meeting or a plan of reformation. No. We are here to walk together with the gaze of Jesus."
'Doubts, confusion, error'
Even before the proceedings began, however, five conservative cardinals asked Francis to reaffirm Catholic doctrine on the treatment of gay couples and the ordination of women.
Their questions -- entitled "Doubts" -- were accompanied by an open letter to followers warning of the risk of "confusion" and "error."
They expressed concern that the questions raised through the Synod process could alienate many Catholics.
In a response made public Monday, Francis appeared to suggest a way to enable the blessing of same-sex couples by clerics, something not recognized by the Holy See but practised in countries such as Germany and Belgium.
While insisting that the Church only recognized marriage between a man and a woman, the pope said that "we cannot be judges who only deny, reject, and exclude," recommending "pastoral prudence."
Since taking office in 2013, Francis has worked to reform the governance of the Church. Striving to make it less hierarchical and more attentive to the needs of the faithful, he has met stiff internal resistance along the way.
Expectations high
For the first time in the history of the Church, nuns and laywomen will take part in the General Assembly consultations and be able to vote.
One informed observer of the Holy See, who asked not to be named, said their participation would make the synodal process more effective.
"Within the bishops, there is an ecclesiastical culture. With the laity, that won't work anymore," the source told AFP ahead of the assembly opening.
"They won't be satisfied with nice words, there will be a demand for [the] procedure, the will to change, efficiency.
"In this sense, Francis is pushing the boundaries, which is why many are afraid."
A second session of the assembly is scheduled for October 2024, meaning that no concrete decisions are expected any time soon.
But expectations are high, as are concerns, with the consultations so far having highlighted differing views between national Churches, and between them and the Vatican.
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