But he said the church doesn't judge and must "seek out and care for hurting couples with the balm of acceptance and mercy."
Francis dove head-on into the most pressing issue confronting the meeting of 270 bishops during a solemn Mass in St Peter's Basilica: How to better minister to Catholic families experiencing separation, divorce and other problems when the church's teaching holds that marriage is forever.
He insisted that the church cannot be "swayed by passing fads or popular opinion." But in an acknowledgement that marriages fail, he said the church is also a mother, who doesn't point fingers or judge her children.
One of the major debates at the synod is whether divorced and civilly remarried Catholics can receive Communion.
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Francis launched the synod process two years ago by sending out a 39-point questionnaire to bishops, parishes and ordinary Catholic families around the world asking about their understanding of and adherence to church teaching on family matters.
Their responses showed a widespread rift between official Catholic teaching and practice, particularly on sex, marriage and homosexuality.
Conservatives insisted that Catholic doctrine is clear and unchanging. Progressives acknowledged the doctrine but sought wiggle room in pastoral practice.
In the ensuing 12 months, both sides have dug in and sparks are expected to fly in Round 2. In fact, few Vatican meetings have enjoyed as controversial a run-up as this one.
There have been allegations of manipulation and coercion; secret caucuses to plot strategy; de-facto laws passed to take the wind out of the debate.
"We are happy if there is turbulence," said Cardinal Lorenzo Baldisseri, the Italian running the synod. "We are in the sea, and so there has to be some turbulence.