In 1965, Paul VI decided on the creation of a joint working group with the World Council of Churches to map all possible avenues of dialogue and cooperation. In the following three years, eight sessions were held which resulted in many joint proposals. It was proposed to work closely together in areas of social justice and development and Third World Issues such as hunger and poverty. On the religious side, it was agreed to share together in the Week of Prayer for Christian Unity, to be held every year. The joint working group was to prepare texts which were to be used by all Christians. On 19 July 1968, the meeting of the World Council of Churches took place in Uppsala, Sweden, which Pope Paul called a sign of the times. He sent his blessing in an ecumenical manner: "May the Lord bless everything you do for the case of Christian Unity." The World Council of Churches decided on including Catholic theologians in its committees, provided they have the backing of the Vatican.
Whose backing did the World Council of Churches require Catholic theologians serving on joint committees to have? (If the question is unanswerable, say "unanswerable")
Vatican