Pope Benedict is presiding over his first major Vatican event, a three-week meeting of more than 250 bishops from around world.
At the synod's opening mass, the pope called for religious belief to play a more prominent role in public life.
The official theme of the gathering centers on a discussion of the Eucharist, or communion. Roman Catholics believe bread and wine consumed during mass is the body and blood of Christ. Bishops are expected to discuss whether Catholics who back abortion rights can take communion.
Other topics include the decline in Mass attendance in some countries and a shortage of priests.
Senior clerics from some 118 countries have gathered in Vatican City, but four Chinese bishops were barred by Beijing from attending. Chinese officials said they were too ill to travel.Some information for this report provided by AP and AFP.