Pope Francis is heading to Sweden today to help start commemorations of the 500th anniversary of Martin Luther's Reformation, a trip that has won him praise from Lutherans but criticism from Catholic conservatives.

Martin Luther started the Reformation in 1517 when he wrote 95 theses criticising the Catholic Church for corruption in Rome, including the buying of ecclesiastical privileges, nepotism, usury and the selling of indulgences.

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.

Francis is thought likely to receive a muted reception in Sweden, which is one of the most secular countries in the world with openly gay Lutheran bishops and cemeteries for atheists. He is traveling 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.

Francis and Lutheran leaders will hold an ecumenical prayer service today in Lund's Lutheran cathedral and another inter-religious event in nearby Malmo.

Last January, Francis, who is keen to continue Catholic-Lutheran dialogue, asked Protestants and other Christian Churches for forgiveness for past persecution by Catholics.

In an interview with Christian Today last week, Bishop William Kenney, the co-chair of the Catholic-Lutheran dialogue, described the visit as "a matter of great symbolic significance". 

"The Lutheran-Catholic dialogue has progressed in leaps and bounds since the 1999 document," said Kenny, referring to the Joint Declaration on the Doctrine of Justification signed between the two Churches. That agreement "removed most of the things that people thought caused the Reformation – the excommunications on the Catholic side, and the condemnations on the Lutheran side – and we said sorry, both sides got it wrong. This left us with the big reasons for the Reformation not being there but it's [about] finding out what that means."

However, Conservative Catholics believe their Church has conceded too much to Lutherans and should not be taking part in any event that praises Luther.

Theological dialogue between Roman Catholics and Lutherans began 50 years ago but they are still officially not allowed to take communion at each other's services.

When he visited Rome's Lutheran church last year, traditionalists attacked Francis for suggesting in answer to a question that a Lutheran woman married to a Catholic man could decide for herself in good conscience if she could take communion in her husband's church.

They also resented that a joint statement when the event was announced in June said the common prayer would "express the gifts of the Reformation".

Pope Francis has called Luther "an intelligent man" who was rightly upset by the corruption, worldliness, greed and lust for power that existed in the Catholic Church at the time.

The deep resentment and rifts left by the Reformation began easing after the 1962-1965 Second Vatican Council urged dialogue with other Christians in the hopes of one day achieving reunion.

Francis has said he wants commemorations of the start of the Reformation to take part in a spirit of humility on both sides.

Although there are still doctrinal disputes he has said both Churches can work jointly on issues such as alleviating poverty and helping migrants and refugees.

Additional reporting by Reuters.