The 81-year-old Argentine will visit the City of Calvin on Thursday at the invitation of the global inter-church organisation, which represents some 350 Protestant, Orthodox and Anglican churches with around 500 million believers among them.
"It is a very important decision he made when he accepted the invitation. (It) says that the Roman Catholic Church has the same agenda as these other churches," said WCC chief Olav Fykse Tveit, a Norwegian Lutheran pastor.
"It is Christian unity in practice," he added, in an interview ahead of the pope's visit to help celebrate the WCC's 70th anniversary.
The trip, he said, was "a sign of hope that even quite deep conflicts and divisions can be overcome through dialogue, through taking one and other seriously."
Historically, divisions between the Catholic Church and the Protestant confessions have indeed run deep.
The dissenting movement launched by Martin Luther more than 500 years ago and its strict interpretation ingrained in Geneva by John Calvin in the mid-16th century launched centuries of often bloody divisions in Europe.
Over the WCC's 70 years, "we definitely see a lot of changes towards openness," Fykse Tveit said, hailing that the time of wars between Christians appears to be over.
He acknowledged though that "it is not difficult to find issues that are still dividing Christians," pointing to attitudes towards "human sexuality and family life".
But he said "there is a kind of momentum for being more united, and the pope's visit is a sign of that".
All Christians shared a common identity as believers, and should be able to rally around the pope's messages of love, tolerance, justice and peace, he said.
"I think that many Christians, whether they are Catholics or not, see him as a strong voice for what we want to say as Christians today."
"We want to ask for justice, work for peace, and give a message of the love of God, of inclusiveness, of giving hope to those who need it."
"In that sense, he speaks for all Christians," he said.
Asked about Catholic doctrine that continues to refer to Protestant denominations as "religious communities" rather than "churches", Fykse Tveit insisted that "in practice, we work together as churches".
While there is generally a growing respect and understanding among Christians, the Lutheran pastor warned that a rival trend was also apparent.
"There is some momentum for being more divided," he said, pointing to disagreements over hot-button issues such as homosexuality and abortion, but also lamenting a growing tendency to mix politics and religion.
Struggle for Christianity's soul
"There is a kind of struggle for the soul of Christianity," he said.
He pointed in particular to the politicisation of Christianity in the United States, where faith has "become a dividing issue", with disagreements over which political camp Christians should choose.
While the WCC does not give Christians guidance on political affiliation, the organisation does insist on the obligation to adhere to true Christian values, Fykse Tveit said.
In the US, the organisation stresses that "all human beings are created equal in the image of God," he said, insisting that all racism is therefore "a basic threat to our Christian faith".
Peacemaking in global crises has been one of WCC's main missions for the past 70 years.
The group has helped counter apartheid in South Africa, pushed for openness in Eastern Europe under Communism, and backed inter-faith dialogue in places like Iraq.
The organisation is currently promoting dialogue through churches in North Korea, and a delegation from the country is due in Geneva for the pope's visit.
Religion today is increasingly on the global agenda, Fykse Tveit said, lamenting though that it is mainly discussed as a problem.
The WCC, he said, had an important role to play in "proving that religion can contribute to positive developments towards peace, justice and human dignity".
Read also: Swiss spills beans about guarding the pope