News that Pope Francis will become the third pope to visit the World Council of Churches when he travels to Geneva on June 21 has led to the natural speculation that he might make the Catholic Church a member of the global ecumenical body.

Since the organization was set up 70 years ago to foster Christian unity, the Holy See has remained outside it, limiting itself to being an observer and often sending representatives to WCC meetings of its 348 Protestant denominations and Orthodox churches.

Successive popes have resisted becoming a member of the body for a variety of reasons, some of which Cardinal Kurt Koch explained to reporters on Friday. The head of the Vatican’s office for promoting Christian unity said the Catholic Church is already a “great ecumenical instrument” which has the conviction — expressed by Pope St. John Paul II “many times” — that the “Petrine ministry is a sign of unity.”

The Swiss cardinal came prepared for the question and referred to the words Blessed Paul VI delivered at the WCC when he visited in 1969.

Catholic membership, Pope Paul said, was not yet “ripe” and he stressed the necessity to further examine the problems posed by such a decision. He didn’t rule it out but said such a step would require “extensive study” along a path which “could be long and difficult.” Such an approach, he added, didn’t prevent the Church from looking to the WCC “with great respect and deep affection,” driven by the pursuit of “the unity desired by Christ.”

The cardinal is of the same mind. More important than membership, he told reporters, is the collaboration that takes place between them. Sitting to his right was the secretary general of the World Council of Churches, Rev. Olav Fykse Tveit, who seemed to agree, saying “we should focus on what we can do together.

The Church and the WCC have worked together on a wide variety of issues: promoting justice and peace, undertaking humanitarian work, and coming together on interreligious dialogue, not to mention cooperating on matters of youth, mission and evangelism — all of which Pope Francis has actively encouraged.

But membership would be highly controversial, as Paul VI inferred. His visit in 1969, and that of John Paul II in 1984, were opposed by some of the faithful who believed such ecumenical gestures promoted a kind of false and syncretic union between the Catholic Church and an alliance of mostly heretical and schismatic sects. The papal visits had no aim to convert them to the one, true faith, they argued, but to falsely portray the ecclesial communities as equal to the Church and thereby affirm them in their heresy and sect.

As then, so today the concern is about what the Second Vatican Council decree Unitatis Redintegratio called “a false irenicism [theology concerned with reconciling different denominations and sects], in which the purity of Catholic doctrine suffers loss and its genuine and certain meaning is clouded.”

It is also largely why the modern ecumenical movement was, at one time, so strongly opposed, not least by St. Maximilian Kolbe. The Polish saint, who gave his life in place of a fellow prisoner at Auschwitz, saw it as the greatest enemy to his Knights of the Immaculata, whose mission was to convert the whole world to the Catholic Church.

For the Church to become a member of the WCC would therefore be unthinkable to many orthodox thinking Catholics. Pope Francis’ predilection for springing surprises, his disregard for convention, and his zealous push for a modern ecumenism — one which critics feel is more about capitulation than conversion — certainly makes membership of the WCC more possible than before, but it remains unlikely.