The Synod of Bishops on the Family runs until 19th October and will host 253 bishops and cardinals from across the world.
Cardinal Vincent Nichols and the Archbishop of Glasgow Phillip Tartaglia will be among them.
The main focus of the discussions is expected to be Catholics who get divorced and remarry.
Currently they are unable to take communion because the Church does not recognise a divorce and considers the person to be living in sin.
There is debate within the Church about how to stop these people from feeling left out of mass.
German Cardinal Walter Kasper has been the most vocal critic of the current rule and says it should be changed so that the Church could "tolerate something that, in itself, is unacceptable".
He has gained moderate support from Cardinal Reinhard Marx, president of the German Bishops' Conference, and Cardinal Oscar Andrés RodrÃguez Maradiaga, chairman of the C9 group of cardinals.
But several figures, including Cardinal Raymond Burke, have criticised this position.
Cardinal Burke, who heads up the Vatican's highest court, the Supreme Tribunal of the Apostolic Signatura,said: "I cannot see how [Cardinal Kasper's idea] can go forward if we are going to honour the words of our Lord himself in the Gospel according to St Matthew, in which he said the man who divorces his wife and marries another commits adultery."
The Archbishop of Westminster, Cardinal Vincent Nichols, said: "This is obviously a crucial part of this debate because it brings together two very significant aspects of Catholic teaching and understanding.
"It brings together a teaching about the indissolubility of a valid marriage...and it also brings together a teaching about the meaning of receiving Holy Communion; that visible sharing in the Eucharist.
"These are two aspects of Catholic faith that have deep roots and deep implications. And I don't see for myself where the area of manoeuvre opens up without quite a radical rethink of one or the other."
Austen Ivereigh from Catholic Voices told Premier the Synod will provide a lively discussion.
He said: "There's a lot of lively disagreement and debate and that will go on in the Synod.
"Pope Francis has said he's not afraid of that disagreement - he thinks it's a good thing.
"He actually believes that through disagreement between people who are basically faithful on key doctrines the Holy Spirit will act."
He added that it was hard to say what the outcome of the Synod could be.
He said: "I think there will be a lot of movement in a lot of directions, but looking at the Synod right now I could not guess.
"I think I can guess on one thing which is annulment reform; Francis has already set up a commission to look at that."
Currently, couples can apply to the Church for an annulment, which allows them to go to communion after separating.
It is expected that this process will become easier.
The leaders will also discuss better pastoral care for Catholics who are struggling with marriage.
A special prayer has been written for the Synod:
Jesus, Mary and Joseph,
in you we contemplate
the splendour of true love,
to you we turn with trust.
Holy Family of Nazareth,
grant that our families too
may be places of communion and prayer,
authentic schools of the Gospel
and small domestic Churches.
Holy Family of Nazareth,
may families never again
experience violence, rejection and division:
may all who have been hurt or scandalized
find ready comfort and healing.
Holy Family of Nazareth,
may the approaching Synod of Bishops
make us once more mindful
of the sacredness and inviolability of the family,
and its beauty in God's plan.
Jesus, Mary and Joseph,
graciously hear our prayer.
Amen