Pope Francis faces the biggest test so far of his Pontificate over the next two weeks as bishops and cardinals from around the world meet in Rome to thrash out their differences over divorce and remarriage.

The extraordinary "Synod on the Family" will ostensibly look at many issues including contraception. But the biggest divide concerns the Church's doctrine that a Catholic divorcee who remarries without obtaining an annulment cannot receive Holy Communion.

This doctrine is based on the biblical teaching that marriage is for life. Its effect however is often perceived unfair and cruel, especially in the Western Church where divorce has become more common.

It means that a devout Catholic woman who is abandoned and divorced by her husband is unable to receive the sacrament at Mass if she remarries without the tortuous process of an annulment.

In the same church, meanwhile, convicted murderers, paedophiles and other criminals who have confessed and received absolution can receive the sacrament with impunity.

More than 250 people are attending the synod, mostly bishops but also 13 married couples.

Among those advocating reform is the German Cardinal Walter Kasper who controversially claimed recently that he "spoke" for Pope Francis when he called for divorced and remarried people to be admitted to communion. Kasper told The Tablet, the international Catholic newspaper, that it was his "impression" that the Pope would like to see an "opening" in allowing divorced and remarried people to receive Holy Communion.

Kasper was immediately attacked by the conservative US Cardinal Raymond Burke, Prefect of the Apostolic Signatura, who described it as "amazing" Kasper could claim to speak for the Pope. Speaking to journalists, Burke said that no matter what his personal view is, even the Pope cannot change Church doctrine on the issue. He attacked Kasper's argument as "fundamentally flawed" and accused him of "misunderstanding" Church doctrine. He also said some of Kasper's comments were "outrageous".

The influential Prefect of the Congregation for the Doctrine of the Faith, Cardinal Gerhard Müller, as well as Australia's Cardinal George Pell have taken the conservative position against reform. Burke and others have argued for the continuing the ban in a new book, 'Remaining in the Truth of Christ'.

According to the veteran correspondent Philip Pullella in Rome, writing for Reuters, the synod, which opens on Sunday, will be "first major showdown" of this papacy. It will be followed by a bigger meeting next year. Besides divorce and remarriage, contraception and homosexuality are on the agenda.

All three areas are known as those where traditional Church teaching is significantly at odds with the practices adopted by Catholics in the pews, as illustrated by the Pope's worldwide survey of Catholics in the run-up to the synod.

Christian Today will from Sunday report daily updates on the III Extraordinary General Assembly of the Synod of Bishops on "The pastoral challenges on the family in the context of evangelisation." The synod will conclude on Sunday October 19 with the beatification of Pope Paul VI, the pope who led and completed the Second Vatican Council.