Cardinal Vincent Nichols, the leader of Catholics in England and Wales is among those attending the event, held the day after a Polish priest revealed he was gay.
#Synod begins today. Please keep me in your prayers these next three weeks.
— Cardinal Nichols (@CardinalNichols) October 4, 2015The Vatican has dismissed Monsignor Krzystof Charamsa but said his sacking was because of the timing of his coming out, rather than his homosexuality.
The 43-year-old - who announced he had a male partner yesterday - had urged the Catholic church to change its stance on gay bishops.
Mr Charamsa told reporters that the timing of his disclosure was not related to the bishops meeting on the family, but said he hoped it might add "a Christian voice" to the synod that is expected to address how the Church can better minister to the homosexual faithful.
"I came out. This is a very personal, difficult and tough decision in the Catholic church's homophobic world," Mr Charamsa said, and asked people to bear this in mind.
Vatican spokesman, Federico Lombardi, said: "The decision to make such a pointed statement on the eve of the opening of the synod appears very serious and irresponsible, since it aims to subject the synod assembly to undue media pressure".
Reporters in Rome said the coming out had set the scene for the three week meeting that many had already expected to be a heated debate.
Up to 300 Catholics will be meeting to discuss family issues, but it's highly unlikely Church doctrine will change.
Traditionalists fear Pope Francis' compassionate attitude to gay and divorced people could be causing confusion about what the Church teaches.
"If a person is gay and seeks God and has good will, who am I to judge?" Pope Francis said in 2013.
