Pope Francis asked forgiveness on Monday for the Roman Catholic Church's "non-Christian and inhumane" treatment in the past of the Waldensians, a tiny Protestant movement the Vatican tried to exterminate in the 15th century.
Francis made his plea during the first ever visit by a pope to a Waldensian temple on the second day of a trip to Italy's northern Piedmont region, the centre of the Waldensian Church, which has only about 30,000 followers worldwide.
While the movement is miniscule compared to the 1.2 billion member Roman Catholic Church, the gesture is part of Francis' drive to promote Christian unity and it has taken on added significance ahead of the 500th anniversary of the Protestant Reformation in 2017.
"On behalf of the Catholic Church, I ask forgiveness for the un-Christian and even inhumane positions and actions taken against you historically," he said. "In the name of the Lord Jesus Christ, forgive us!"
The Waldensians, who now live mostly in Italy and Latin America, were founded by Peter Waldo in France in the late 12th century. He gave up his wealth and preached poverty but as the movement grew it came into increasing theological conflict with the papacy.
The movement, an early precursor of the Protestant Reformation in the 16th century, was branded as heretical and in 1487 Pope Innocent VIII ordered its extermination.
Some 1,700 Waldensians were killed in 1655 by Catholic forces commanded by the Duke of Savoy.
Today the Waldensians are part of the World Communion of Reformed Churches and the World Council of Churches. They have
only two sacraments -- baptism and the Lord's Supper -- as opposed to seven in the Roman Catholic Church, a member of the Italian branch said.
During a visit to Jerusalem in 2000, Francis' predecessor Pope John Paul II asked forgiveness from Jews for their persecution by Catholics over the centuries.
In 2017, Christians will mark the 500th anniversary of the launching of the Protestant Reformation by Martin Luther, who nailed his 95 Theses to a church door in Germany to denounce corruption in the Catholic Church.
Various Christian churches have still not agreed on how they will jointly mark the divorce that split western Christianity and led to many bloody religious wars.
The head of the Protestant Church in Germany, Bishop Heinrich Bedford-Strohm, has said he would be in favour of inviting the pope to a "joint Christian festival" to mark the anniversary.
Francis, an Argentinian whose father and grandfather were born in the Turin area, wrapped up his two-day visit to the region with a private lunch with his Italian relatives.