Systematic reflection on the principles and postulates of the ecumenical movement, notably among Protestant and Eastern theologians. They hope to find the way that leads beyond mere co-operation to a true unity that will make it clear to the whole world that, as there can only be one Body of Christ, so there is only on Body which is the church of his people. The basic problem therefore, is ecclesiological.
At one extreme are theologians who believe there is strength in doctrinal divergence, for whom the ecumenical movement should lead only to a federated co-operation among the churches with no ambition to organic unity. they are "keenly sensitive to the gains in vitality" that come from Church differences. At the other extreme are Eastern churchmen for whom the unity of the Church already exists and in fact is to be found within the exclusive limits of their own communion.
Between these extreme lie the majority of Protestant and Orthodox positions in the ecumenical movement. they are undecided either on the nature of the Church or on the kind of unity it is supposed to have. some maintain that "the unity of the Catholic (not Roman) church is an existing historic reality" within certain theoretical boundaries. Their problem is in defining these boundaries, within which the Church may be united and beyond which diversity is allowed.
Others believe that the Church is a purely invisible entity, a community known only to God. Its unit, therefore, is also known only to him, and the task of theology is to give better expression to this existent--so far mostly invisible--unity among the divided members.
Still others hold that the Church is essentially all those who profess and call themselves Christians, however diverse their belief and practice. This seems to be the majority opinion in the present World Council of Churches.
The Roman Catholic Church has not been indifferent to the efforts of scholars outside her ranks to reunite a dismembered Christian world. Catholic theologians have promoted the most extensive study of church unity since the Reformation. While holding firm to their conviction that unity is possible only through union with the See of Peter, they stress the sincerity of ecumenical efforts outside of Rome and the presence of the Holy Spirit in such deliberations.