Catholic bishops are congregating in Rome for three weeks of debate on the family that could radically reshape the Church's approach to the divisive issues of divorce and homosexuality.
A second and final round of a review of Church teaching on a broad spectrum of questions related to family life opens on Sunday - and Vatican officials are being unusually frank in admitting that clashes are inevitable.
"We are at sea, in choppy waters even," said Lorenzo Baldisseri, the cardinal who serves as Secretary General of the synod, as the assembly of bishops is known.
Although battlelines are not always clearcut, the most important division is between reformers who want to make the Church more accommodating towards gay, divorced or cohabiting believers, and those opposed to any dilution of centuries-old doctrine which holds that marriage is for life, full stop, and that homosexuality is sinful or an abomination.
Pope Francis has made it clear he favours a fresh approach and his status as the infallible Vicar of Christ means he can ultimately decide what he likes.
But most Vatican experts agree he will be looking for majority support for any changes.
Last year's first round of discussions made global headlines when early draft conclusions included a passage saying the Church should recognise the value of loving, same-sex relationships - a groundbreaking formula that had echoes of Francis' own "who am I to judge?" soundbite on the issue.
But a backlash from bishops who claimed they had never been given a chance to discuss the controversial passage ensured it was removed from the final text.