[1] In modern times, the expression "sister Churches" first appeared in John XXIII's letters to the Orthodox Patriarch of Constantinople, Athenagoras I.
In addition, the lifting of mutual excommunications, by eliminating a painful canonical and psychological obstacle, was a very significant step on the way toward full communion.More recently, the joint international commission took a significant step forward with regard to the very sensitive question of the method to be followed in re-establishing full communion between the Catholic Church and the Orthodox Church, an issue which has frequently embittered relations between Catholics and Orthodox.
This is not merely a question of terminology, but above all of respecting a basic truth of the Catholic faith: that of the unicity [uniqueness] of the Church of Jesus Christ.
Finally, it must also be borne in mind that the expression 'sister Churches' in the proper sense, as attested by the common Tradition of East and West, may only be used for those ecclesial communities that have preserved a valid episcopate and Eucharist [i.e., as indicated in beginning of document, it is 'improperly applied' when referring to 'the Anglican Communion and non-Catholic ecclesial communities'].
He specifically mentioned obstacles like "lay presidency, the ordination of women, and ethical problems such as abortion and homosexual partnerships."
Kallistos Ware notes: "For Orthodoxy, the validity of ordinations does not depend simply on the fulfillment of certain technical conditions (external possession of the apostolic succession; correct form, matter and intention).
[6] The Catholic Church seems to be of the same mind concerning broader and "more substantive" criteria (not merely "the fulfillment of technical conditions") necessary for recognition of Anglican orders.
According to Rosemary Radford Ruether, it unhappily suggests that the "only ecumenism" that "counts" is "between Rome and Constantinople", though Anglicans, a second millennium church, "yearn to get into the 'old boys' club' of patriarchs.