Pope Benedict opened a synod of more than 250 bishops from 118 countries during a solemn Mass in St Peter's Basilica this morning.
The official theme of the synod is the sacrament of the Eucharist, which Catholics believe is Christ's body and blood.
But discussion is expected on issues such as the shortage of priests, the decline in Mass attendance and whether Catholics who back abortion rights should be able to receive communion.
The US Catholic community was divided last year over whether they should support presidential candidate John Kerry, himself a Catholic who supported abortion rights.
Some Catholics say they personally would not have an abortion but, in pluralistic societies such as the United States, feel obliged to support a woman's right to choose.
But the synod's working document says Catholics cannot back politicians who openly support abortion.
There also have been legal conflicts in the United States and Italy about whether religious symbols such as crucifixes and the tablets of the Ten Commandments can stay in public places.
The three-week-long synod will offer advice to Benedict, who is expected to attend all or most of the sessions, giving him a chance to meet many of the bishops for the first time.
In another part of the homily, the Pope elaborated on the Biblical story of the vineyard which produced bad grapes.