The summit that ends on October 25 and is known as the synod, started on Sunday with a mass in St Peter’s Cathedral.
It is not expected to change church doctrine, but to discuss how its teaching can be adapted to modern lifestyles.
The most divisive issue is whether a longstanding ban on remarried divorcees taking communion could be dropped.
Several cardinals on both sides of the argument have warned that the Vatican risks a schism over the controversy as church attitudes toward same-sex couples remained a flashpoint issue.
On Saturday, the Vatican was rocked by the decision of one of its theologians who told an Italian newspaper that he was gay and living with another man.
Poland-born Monsignor Krysztof Charamsa was stripped of his duties after his admission.
Vatican spokesman, Father Federico Lombardi, called his move “very serious and irresponsible” because it placed synod participants under “undue media pressure”.
Opening the three-week summit on Sunday, Pope Francis defended marriage as an unbreakable bond between man and woman and said the church should not be influenced by “passing fads or popular opinions”.
But the pontiff also called for a merciful church, capable of being “a good Samaritan to wounded humanity,” and said that a “closed doors” church “betrays herself and her mission, and, instead of being a bridge, becomes a roadblock”.