Pope Francis has launched a two-year worldwide consultative process that could change the way the Roman Catholic Church makes decisions and leave its mark long after his pontificate is over.
Some Catholics hope it will lead to change on issues such as women's ordination, married priests and same-sex relationships.
Conservatives say the three-stage process is a waste of time, may erode the hierarchical structure of the 1.3 billion member Church and in the long run could dilute traditional doctrine.
At a Mass in St. Peter's Basilica, Francis said Catholics should have a open mind about the process.
"Are we prepared for the adventure of this journey? Or are we fearful of the unknown, preferring to take refuge in the usual excuses: 'It's useless' or 'We've always done it this way'"? The Pope said in his homily.
The consultation process, called "For a Synodal Church: Communion, Participation and Mission", will work in three stages:
In the first stage, Catholics in parishes and dioceses around the world will discuss issues such as whether the Church listens enough to young people, women, minorities, and those on the margins of society.
Francis has said that while there should be more consultation and mutual listening among the various parts of the Church, only popes can take the ultimate decisions on doctrinal matters.
The "continental phase" will see bishops gather to discuss and formalise their findings.
The "universal phase" will see a month-long gathering of the bishops a the Vatican in October 2023.
The Pope is expected then to write an apostolic exhortation, giving his views and decisions on the issues discussed.
Discussing his hopes for the Synod, Pope Francis warned against the process becoming an intellectual exercise that fails to address the real-world issues faced by Catholics and the "temptation to complacency" when it comes to considering change.
Meanwhile, the progressive US-based National Catholic Reporter newspaper praised the initiative as a chance for more inclusiveness.
"It may not be a perfect process, but the Church is more likely to address the needs of the people of God with it than without it," an editorial said.