While the majority of the founding members came from Europe and North America, currently the bulk of the WCC membership is in Africa, Asia, the Caribbean, Latin America, the Middle East and the Pacific. The Holy See is not a member of the WCC, but it is an observer, and routinely sends representatives to the organization's meetings.
Francis' homily during the prayer gathering was the first official speech of his daytrip to Geneva. He spoke at the WCC headquarters after holding a private meeting with President of the Swiss Confederation, Alain Berset.
In his address, the pope said Christian divisions have historically arisen because " a worldly mindset has seeped in" at their root.
What happened, he said, is that "self-concern took priority over concern for Christ," and once this took place, devil "had no difficulty in separating us, because the direction we were taking was that of the flesh, not of the Spirit."
Even certain attempts to end these divisions in the past have "failed miserably because they were chiefly inspired by a worldly way of thinking," he said, noting that the ecumenical movement "came about as a grace of the Holy Spirit."
"Ecumenism made us set out in accordance with Christ's will, and it will be able to progress if, following the lead of the Spirit, it constantly refuses to withdraw into itself."
Looking at relations between modern Christian churches and the slew of issues which often stand in the way of full unity, Francis said the current experience is akin to that of the early Christian communities in Galatia.
"How difficult it is to overcome hard feelings and to foster communion! How hard it is to leave behind centuries-old disagreements and mutual recriminations!" he said.
At times, it is "more formidable to withstand the subtle temptation to join others, to walk together, but for the sake of satisfying some partisan interest." However, this is not the mindset of an apostle, but is the attitude of Judas, who walked alongside Jesus, "but for his own purposes."
The 70th anniversary of the WCC, Pope Francis said, is a call to strengthen the steps toward ecumenism that have already been taken.
He said Christians should not cease their quest for unity when faced with continual differences, and nor should they be overcome by weariness or a "lack of enthusiasm."
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"Our differences must not be excuses. Even now we can walk in the Spirit: we can pray, evangelize and serve together," he said. "This is possible and it is pleasing to God! Walking, praying and working together: this is the great path that we are called to follow."
The aim of this path is unity, and the opposite is a path to division which leads to "conflict and breakup," he said, stressing that the lack of unity among Christians is not only "openly contrary to the will of Christ," but is also "a scandal to the world and harms the most holy of causes: the preaching of the Gospel to every creature."
The Lord, he said, "asks us for unity; our world, torn by all too many divisions that affect the most vulnerable, begs for unity."
And for Christians, to walk together is not merely a "ploy to strengthen our own positions," but is rather an act of obedience to Jesus and his love for the world, Francis said, and closed by praying that God would help Christians to "walk together all the more resolutely in the ways of the Spirit."
"May the Cross guide our steps, because there, in Jesus, the walls of separation have already been torn down and all enmity overcome."