During his installation service, the Rev Dr Olav Fykse Tveit pronounced "Nothing except Jesus Christ, and Him crucified" as his first call of unity to the worldwide ecumenical fellowship.
He said the cross was the symbol that churches needed to return to in order to be one.
"The cross is and will forever be the sign of the church. This is the symbol that we have together, the symbol of what we have together, the symbol of what the churches have to give to the world. From the beginning to the end."
Just before his sermon, Tveit acknowledged to the media on Monday that there were "a lot of unsolved issues" among WCC member churches and denominations, which are located in more than 110 countries and territories and represent 560 million Christians around the world.
The role of the World Council of Churches, he said, was to assess where they are today, balance tensions, and determine where they should go in the future.
How ecumenical relations would develop was not in their hands, but in God's, Tveit noted.
But what is in their hands is the ability to commit themselves to finding "theological, ethical and very visible expressions" of being one, he said.
Addressing representatives of the ecumenical body in Geneva, the newly installed WCC general secretary called them to move forward together with the perspective of the cross.
"The cross is the perspective that must not be missed out," said Tveit, who was elected last August.
Pointing to the image of Christ crucified with his arms stretched out, he called the ecumenical body to take on that same "inclusive" image, embracing everybody and sharing God's gifts with all.
"The cross is the first and ultimate sign of the gift of God’s gracious being with us and for us," he said. "We are one as Christians because we receive the same gift. That is why there is nothing except the cross."
Tveit also identified the cross as the "reality check" of the churches, the ecumenical movement and their faith. The cross "proves everything", from the power of humility to faithfulness to God, he noted. It also "tests everything in our will to follow Jesus Christ".
"So we are called to carry our cross," he underscored.
"The call of the ecumenical movement does not have a meaning only if we succeed. Whether we are heard or not, our call is to carry the cross with one another.
"Whatever happens, it remains our call to carry the cross in our search for unity, in our witness, in our service. And we shall do it together, never alone."
Tveit is one of the youngest men to serve as general secretary of the World Council of Churches.
Since his election, he has placed weight on the call for church unity, not only within the WCC but also outside of the body with other groups such as the Roman Catholic Church, the Pentecostal churches and evangelical bodies.
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