Vatican City ? Bishops from around the world begin tackling major issues facing the Roman Catholic Church on Sunday, including whether Holy Communion should be given to Catholic politicians who back abortion rights and to divorced people who remarry without getting an annulment.

The priest shortage, and whether celibacy contributes to it, as well as dwindling Mass attendance also are expected to be discussed at the three-week Synod of Bishops, during which bishops will offer recommendations to Pope Benedict XVI on running the church.

The Vatican said Saturday that 256 bishops from 118 countries would participate – the most ever. In addition, 10 representatives from non-Catholic Christian churches will attend, although they do not have voting rights.

Benedict formally opens the synod with a Sunday Mass, and in many ways the meeting is as much about him as it is about the bishops since it is the pontiff’s first major Vatican undertaking since being elected April 19.