Speaking at an annual vespers service in St. Paul's Basilica in Rome attended by representatives of other religions, he said, "Let us ask, above all, forgiveness for the sin of our divisions."

The Vatican announced that on October 31, Francis would go to the southern Swedish city of Lund, where the Lutheran World Federation was founded in 1947, for a joint service with Lutherans to launch Reformation commemorations that will continue throughout the world next year.

Martin Luther, a German, is credited with starting the Protestant Reformation in 1517 with writing 95 theses - said to have been nailed to a church door in Wittenberg - criticizing the Catholic Church for selling forgiveness from sins for money.

It led to a violent, often political schism throughout Europe and Christianity, prompting among other things the 30 Years' War, the destruction of English monasteries, and the burning of numerous "heretics" on both sides.

Catholic traditionalists have accused Francis of making too many concessions to Lutherans, particularly in a "common prayer" that both religions will use during the 2017 commemorations.

They say the prayer, which will be used during the pope's visit to Lund, excessively praises Luther, who was condemned as a heretic and excommunicated.

Francis, however, has made dialogue with other religions one of the hallmarks of his papacy.

He has already visited the Lutheran church of Rome, the Waldensian protestant community in northern Italy, and Rome's synagogue. This year he is due to become the first pope to visit the Italian capital's mosque.

While his predecessors have visited Protestant churches, Francis has come under criticism from traditionalists who accuse him of sending confusing signals about inter-faith relations.