MALMÖ, Sweden: Pope Francis arrived in Sweden Monday on a historic mission to promote reconciliation and unity between Catholics and Protestants, riven by Christianity's nearly 500-year-old schism.

"It is an important trip," the pontiff told journalists on a plane carrying him from Rome to Sweden, a predominantly Lutheran nation hosting commemorations to mark the birth of Protestantism.

After arriving in the southern city of Malmo, Francis was to head to nearby Lund for an oecumenical service marking the start of a year of celebrations for the Reformation - the dramatic 1517 event that created a Protestant branch of Christianity which rebelled against papal rule.

The event will also mark 50 years of reconciliatory dialogue between the Catholic Church and Lutheranism - a Protestant branch that has traditionally been among the most fervent opponents to Vatican's authority and teachings.

Just by agreeing to attend, Francis has made a gesture that would have been unimaginable for all but his most recent predecessors, commentators say.

The popes of the 16th century spent huge amounts of time and energy trying to stifle or reverse the reforming wave launched by the German monk Martin Luther when he nailed his demands - the "95 theses" - to the door of a church in Wittenberg on October 31, 1517.

Monday's meeting comes eight months after Francis became the first pope in almost 1,000 years to meet an Orthodox Patriarch.

The current leader of the world's 1.2 billion Catholics has also reached out to Anglicans.

Ahead of the visit, Francis reiterated the importance he attaches to Christian unity at a time when both believers and belief itself are under pressure in many parts of the world.

"When Christians are persecuted and murdered, they are chosen because they are Christians, not because they are Lutherans, Calvinists, Anglicans, Catholics or Orthodox," Francis said in an interview with two Jesuit publications.

"An oecumenism of blood exists."

Some Catholic conservatives question whether there is anything about the Reformation worth celebrating.

Partly for that reason, every word of the sermon the ever unpredictable Francis delivers in Lund is likely to be closely scrutinised, as will remarks by Mounib Younan, the Palestinian president of the World Lutheran Federation.

The pontiff's body language will also be closely watched, particularly when he is led into the Lund cathedral by Antje Jackelen, the female archbishop who is the senior cleric in the Swedish Lutheran church.

"It is clear that in order to recognise priests within another church, this is not an advantage because it is not possible for us to have women priests," Kurt Koch, president of the Pontifical Council for Promoting Christian Unity, told the Swedish daily Dagens Nyheter on Monday.

"We accept of course the decision about having a woman archbishop. But it is an obstacle in the way of recognising the other church's tradition," Koch said.

Antje acknowledged, without touching on the issue of women's role in the church, that it has been complicated to organise the event in Sweden.

"It has not always been easy, partially because it is a completely unique experience and partially because there are so many chefs involved," Jackelen told Dagens Nyheter.

With its approval of women holding office, backing for gay marriage and openly lesbian and gay bishops, the Swedish church is liberal to an extent unimaginable for the vast majority of Catholic clerics.

The two traditions also differ in their approach to church governance - hierarchical for Catholicism, flat for the Lutherans - as well as on more esoteric theological questions.

But they also share many religious customs, such as baptism, and Younan told AFP he would like to see Catholics and Lutherans authorised to take communion together - something currently ruled out by Vatican doctrine.

"We are praying that one day we may celebrate the holy communion together, this is very important for me," Younan said, while stressing the importance of accentuating common ground.

"In this time when extremism is devouring all the world globally, we are giving an example to the whole world that this a common commemoration despite our disagreement in the past, a sign of unity and a sign that religion is no more a problem."

Monday's programme also includes an event in a stadium in Malmo that will be addressed by the bishop of the besieged Syrian city of Aleppo. It will conclude with a mass prayer for peace in the war-torn country. The charity wings of the two churches are also due to seal a cooperation accord.