Pope Francis is traveling to Sweden to join Lutheran leaders for a joint commemoration of the 500th anniversary of the Protestant Reformation.

The anniversary of monk Martin Luther's challenge to Catholic dogma may not seem like an event to celebrate from the Vatican's perspective, but Francis' participation is part of the Vatican's wider efforts to mend ties with other Christians.

For history's first Jesuit pope, the visit is particularly significant given the Jesuits were founded to defend the Catholic faith from Protestant reformers.

Here are some questions and answers about the pope's visit:

Q: WHAT WAS THE REFORMATION?

A: The Protestant Reformation started in 1517 when German theologian Martin Luther nailed 95 theses on a church door in the town of Wittenberg, denouncing what he saw as abuses of the Catholic Church. Luther objected to the practice of selling indulgences to reduce the punishment for sins and challenged the pope's authority.

Pope Leo X excommunicated Luther, but the church couldn't stop his teachings from spreading throughout northern Europe. Religious wars erupted, including the Thirty Years War in 1618-48, one of Europe's bloodiest conflicts.

Protestantism became one of three major forces in Christianity, together with Roman Catholicism and Orthodoxy. Lutheranism is one of the main Protestant branches.

___

Q: WHY WOULD FRANCIS WANT TO COMMEMORATE IT?

A: One of Francis' main priorities as pope has been to promote what he calls a "culture of encounter" in which people of different faiths, especially Christians, walk, talk and pray together.

He joined the spiritual leader of the world's Orthodox Christians to meet with refugees in Lesbos, Greece. He prayed for martyred Catholics and Anglicans in Uganda. And he asked forgiveness for the Catholic Church's persecution of the small evangelical Waldensian Church in Italy.

He says he wants to bring that message of "coming together" to Sweden and its Lutheran Church.

Not all his advisers approve, however.

The Vatican's doctrine czar, German Cardinal Gerhard Mueller, said in a book interview that Catholics have "no reason to celebrate" the anniversary "which brought about the fracture of Western Christianity."

___

Q: DO CATHOLICS AND LUTHERANS HAVE GOOD RELATIONS?

A: Following the Reformation, Protestants were denounced as heretics and persecuted in Europe's Catholic countries.

The Lutheran Scandinavian countries, on their part, enacted strict anti-Catholic laws to prevent their former faith from making a comeback. A ban on Catholic convents in Sweden wasn't lifted until 1977.

Dialogue between the Vatican and the Lutherans improved relations in recent decades and led to a landmark 1999 joint declaration on the doctrine of justification concerning God's forgiveness of sins. That had been one of the main stumbling blocks in their relations.

Theological divisions remain, however, and Francis is using the trip to encourage other ways the two churches can work together, particularly on humanitarian initiatives.

___

WHAT ABOUT THE POPE'S OWN HISTORY WITH LUTHERANS?

A: As rector of a Jesuit seminary in Argentina, the former Jorge Mario Bergoglio gave a 1985 speech 1985 in which he tore to shreds the theology and philosophy underlying Luther and Calvin, denouncing the heresy and schism that resulted and which his Jesuit order was founded to fight. He was chastised by Lutheran friends for other "offensive" comments.

He has since changed his tune.

This summer, Francis told reporters Luther was a reformer whose intentions weren't wrong since the Catholic Church of the time was "corrupt, worldly, attached to money and power."

The pope has gone so far with his own reform agenda that conservatives accuse him of "Protestantizing" the Catholic Church.

___

Q: WHAT WILL THE POPE BE DOING IN SWEDEN?

A: Francis' main event is an ecumenical service Monday with the Lutheran World Federation at the Lund Cathedral to commemorate the Reformation and give thanks for improved relations. Afterward, he travels to nearby Malmo, the largest city in southern Sweden, for another ecumenical event featuring testimonies by refugees as well as the bishop of besieged Aleppo, Syria.

On Tuesday he will preside over Mass in Malmo's soccer stadium in front of more than 15,000 people. A Lutheran delegation plans to attend.

Why Lund, population 80,000? The Lutheran World Federation was founded there in 1947.

___

Q: HOW BIG IS SWEDEN'S CATHOLIC COMMUNITY?

A: The Catholic Church counts about 113,000 members in Sweden, the most since the Reformation. Most are migrants, though about 100 Swedes convert each year.

Francis originally planned a one-day trip for the Lutheran event and resisted doing anything special for local Catholics to preserve the ecumenical nature of the visit. But after the Catholic community protested, Francis scheduled a second day so he could celebrate a public Mass, even though it meant missing a major feast day in Rome.

The most famous Swedish Catholic is St. Bridget, the 14th century mystic who founded the Bridgettine order. But Sweden's latest saint has particular relevance for the visit: St. Elizabeth Hesselblad, a Lutheran convert to Catholicism who hid Jews in her Rome convent during World War II. Francis canonized her in June.

___

Winfield reported from Rome.