I met the Right Reverend Archimandrite Robert Taft, S.J., at an Eastern-rite monastery that I was visiting in 1985. The community was still in the refectory, whereas I happened to be near the vestibule, so I was the one who went to the front door when he rang. There was a moment of confusion: I had had no idea that the monks were expecting such a renowned guest, and the guest may have expected a more formal reception. Yet it was fitting that a Jesuit scholar of the Byzantine liturgy should be greeted by a “porter” whose father was Ukrainian Catholic and whose mother was of the Latin rite.
With all due respect to Abouna [Father] Robert, who for decades has served the Catholic Church well as an erudite scholar and a tireless ecumenist, he insistently uses the expression “Sister Churches” in a way that could easily be misleading in his recent interview with Catholic World Report. The editor helpfully linked the expression to a page that thoroughly explains the significance of “particular Churches” in post-Vatican-II ecclesiology. For those who have neither the patience nor the theological training to synthesize the wealth of information on that page, this blog post may help clarify the matter.
The Catechism of the Catholic Church begins its teaching about the article of faith from the Creed, “I believe in the holy catholic Church,” with a few notes on terminology. “In Christian usage, the word ‘church’ designates  the liturgical assembly, but also  the local community or  the whole universal community of believers. These three meanings are inseparable” (CCC 752). In everyday conversation we move easily and without confusion among these different meanings. “We went to church this morning .” “I’m registered at the Church of the Annunciation .” “Christ promised to be with His Church always .”
Because a diocese is normally headed by a bishop, who has the fullness of Holy Orders, while a parish is usually headed by a priest, in theological discussion the second usage of “Church” usually refers to a “local Church” or a “particular Church”. In the Latin rite this is called a diocese or an archdiocese; “eparchy” and “archeparchy” are names for it in the Byzantine rite. The relations between this “mid-sized” Church  and the other two connotations of “Church” can be discerned in the New Testament and are stated clearly as early as the second century in the Letters of Saint Ignatius. The local Church exists—for example, in Philadelphia or in Ephesus—for the sake of liturgical worship, which inaugurates and sustains the life of grace in Christians; moreover the Eucharist and even the sacrament of marriage is always to be celebrated in union with the local bishop (i.e. with his approval if he does not actually preside). The connection between the local Church  and the universal Church  is evident in Ignatius’ insistence on the unity of faith and the reality of Christ’s [Mystical] Body.
The expression “Sister Churches” is not theological but historical and (in recent years) diplomatic. Fr. Adriano Garuti, O.F.M., a professor of ecclesiology and ecumenism who has served with the Congregation for the Doctrine of the Faith, writes: “The intention behind such language is the establishment of the reality of sister Churches as a possible way to ‘envisage reunion among divided traditions as a family reconciliation’…. One does get the impression, however, that a certain ambiguity and lack of continuity prevail in the use of the term.” The uses and misuses of this expression are examined in depth in his essay “Sister Churches: Reality and Questions” (reprinted in the book Primacy of the Bishop of Rome and the Ecumenical Dialogue by the same author.)
The early Church in the East was organized not only by locality but regionally. A “Metropolia” united several local eparchies and/or archeparchies in an administrative unit. Within such a unit, two neighboring eparchies would be regarded as “Daughter Churches” of the Metropolia and therefore “Sister Churches” to one another. Fr. Garuti notes “the special sensibility of the Eastern Christians for the fraternity that exists among the individual [local] Churches ”. He immediately goes on to add, however, that “when it is a question of the principles on which to build unity, … the [Universal Catholic] Church  cannot be considered a sister [e.g. to the Orthodox Churches (2)], but rather the Mother of the local Churches.”
When Pope Francis referred to himself as “the Bishop of Rome” in his first public speech on the balcony of St. Peter’s Basilica, he was humbly acknowledging that in the first place he had been elected the Bishop of Rome, a local Church. As Bishop of Rome he can greet Orthodox bishops of other localities as “brother bishops”, since they head “sister Churches ”. But the Bishop of Rome is also ex officio the Pastor of the Universal Church , and there is no corresponding office or “unit” in the Orthodox world, nor could there ever be.
Joseph Ratzinger pointed this out as early as 1966, just after the completion of the Second Vatican Council. At a Catholic Conference in Bamberg he urged caution when speaking about “the Churches” in the plural, warning against “a euphoria … that forgets to makes difficult demands on itself and overlooks the fact that the Catholic Church dares and must dare to take the paradoxical position of attributing to herself in a unique way the singular form, ‘the Church’ , despite and in the midst of the plurality  she has accepted.” (Quoted in Joseph Ratzinger: Life in the Church and Living Theology by Maximilian Heinrich Heim.) Because it can lead to misunderstandings between Catholics and Orthodox, Joseph Ratzinger scrupulously avoided the expression “Sister Churches” in his extensive writings on ecumenism.
In conclusion: the Right Rev. Archimandrite Robert Taft is not the only ecclesiologist on the block. If he had used the expression “particular Churches” in his interview, he would have been more accurate, because that (and not “Sister Churches”) is the expression that has been enshrined in the Catechism and in post-conciliar Catholic ecclesiology.