Speaking to bishops from around the world, he has stressed that marriage is a binding bond between a man and woman.
Emphasisng the church does not judge he said it must: "seek out and care for hurting couples with the balm of acceptance and mercy".
270 bishops attended the Mass in St. Peter's Basilica.
During the discussion - How to better minister to Catholic families experiencing separation, divorce and other problems when the church's teaching holds that marriage is forever, the Pope said that the church cannot be "swayed by passing fads or popular opinion".
He did admit that sometimes marriages do not work though and added that the Church is itself a mother who does not point fingers or judge her children.
Francis said: "The church must search out these persons, welcome and accompany them, for a church with closed doors betrays herself and her mission, and instead of being a bridge becomes a roadblock."
A number of issues will be discussed over the three weeks, however one of the major debates will be whether divorced and civilly remarried Catholics can receive Communion.
Francis began 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 widening gap between official Catholic teaching and practice, particularly on sex, marriage and homosexuality.
Last year there was a meeting of bishops on similar matters but they failed to reach a consensus on how to better welcome gays and divorced and civilly remarried Catholics in the church.
Conservatives in the Church think that Catholic doctrine is clear and unchanging, but liberals say there is room for movement in pastoral practice.
Cardinal Lorenzo Baldisseri, the Italian running the synod commented: "We are happy if there is turbulence.
"We are in the sea and so there has to be some turbulence."
But Cardinal George Pell, the Vatican's finance manager, predicted little would change and the bishops would reaffirmation of the status quo.
Former Irish president Mary McAleese, a practicing Catholic with a gay son, said she hoped that more transparency would help "kill for once and all this terrible lie" that everyone was born heterosexual.
