Pope Benedict has opened a meeting of Roman Catholic bishops from around the world, the first under his pontificate.
The Synod is Pope Benedict's first major meeting since his election
During the three-week synod, bishops are expected to discuss a range of controversial issues with the new Pope.
Church attendance, divorce, celibacy and abortion rights are all expected to feature on the synod's agenda.
The BBC's David Willey in Rome says dwindling numbers of priests and shrinking congregations are particularly worrying for the Vatican.
Speaking at the synod's opening Mass in St Peter's Basilica, Benedict XVI said religion should play a role in public life.
"A tolerance which allows God as a private opinion but which excludes Him from public life... is not tolerance but hypocrisy," the Pope said in the homily.
"When man makes himself the only master of the world and master of himself, justice cannot exist."
The increasing shortage of priests in the Catholic Church was highlighted in a document circulated in advance of the meeting.
There was one for every 2,677 Catholics in 2003 - the figure was one in 1,797 in 1978.
Only 5% of Catholics attend Mass in some countries, the document added.
Our correspondent says some discussion of the Church's celibacy rule is not ruled out.
Some Catholics believe more men would join the priesthood if they were allowed to marry.