Francis urged a spirit of "solidarity, courage and humility" as the Catholic Church's conservative and liberal wings began tackling hot-button topics from communion for remarried divorcees to acceptance of homosexuality.
The 78-year old reminded an audience packed with red- hatted cardinals and purple-sashed bishops that the Church was "not a museum to keep or preserve.
"It's a place where the holy people of God move forward," he told an audience that included a baby whose parents were among 18 lay couples participating in the synod.
In an apparent boost to those who would have priests take a more merciful and inclusive approach to homosexuals, he said the Church must not "point the finger in judgement" of others, because doing so would "betray its mission".
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But he has also said traditional marriage between heterosexual couples should be defended.
"The emotional intensity of the synod is amped up because of perceptions that the pope's position is still a work in progress," Vatican watcher John Allen said.
"What's new now is a sense, however exaggerated, that movement might actually be possible. That's elicited strong passions both from those who see such movement as desirable, and those who view it as alarming."
The synod is the second and final round of a review of Catholic teaching on the family.
During last year's first round, language that would have seen the Church recognise the value of loving same-sex relationships was excised from the final working document that forms the basis for the talks here.
The gathering of 360 participants was overshadowed this weekend by the bombshell announcement by a Polish priest, Krzystof Charamsa, an official at the Vatican, that he is a practising homosexual with a partner.