Pope John Paul II implored Christians on Sunday to overcome the "painful trial" of their divisions, aggravated by such issues as the election of a gay Episcopal bishop and the hostility between Catholic and Orthodox faithful in eastern Europe.
Greeting pilgrims in St. Peter's Square, John Paul noted that Sunday was the start of an annual week of prayer dedicated to Christian unity.
"During the next eight days, in every part of the world, Christians of different confessions and traditions will gather to pray intensely that the Lord strengthen the common commitment for their full unity," John Paul said.
This year's theme — peace — was chosen by churches in the Middle East.
"In promoting his peace, Christ assures his disciples support in their trials. And isn't the lasting division among Christian perhaps a painful trial?" said the pope, who has made efforts to encourage unity among Christians a goal of his papacy.
He said Christians feel the need to pray for help "to overcome the temptation of discouragement along the difficult path which leads to full communion."
"In a world craving for peace, it is in fact urgent that Christian communities announce the Gospel in a united way," John Paul said.
The election last year of the first openly gay bishop in the U.S. Episcopal Church deepened divisions among Anglicans and complicated efforts to unify the Roman Catholic and Anglican churches. The bishop's ordination also caused tensions between Orthodox Christian and Anglican faithful.
In another source of frustration for the pope, Roman Catholics and Orthodox have been arguing over property interests since the collapse of Soviet bloc communism. Orthodox also accuse the Vatican of aggressively seeking converts in the region.
The friction has made it impossible for John Paul to visit Russia, one of the few major countries that has eluded the pontiff in 25 years of global travel during his papacy.