From RESPONSES TO SOME QUESTIONS REGARDING CERTAIN ASPECTS OF THE DOCTRINE ON THE CHURCH:
FOURTH QUESTION
Why does the Second Vatican Council use the term “Church” in reference to the oriental Churches separated from full communion with the Catholic Church?
RESPONSE
The Council wanted to adopt the traditional use of the term. “Because these Churches, although separated, have true sacraments and above all – because of the apostolic succession – the priesthood and the Eucharist, by means of which they remain linked to us by very close bonds”[13], they merit the title of “particular or local Churches”[14], and are called sister Churches of the particular Catholic Churches.[15]
“It is through the celebration of the Eucharist of the Lord in each of these Churches that the Church of God is built up and grows in stature”.[16] However, since communion with the Catholic Church, the visible head of which is the Bishop of Rome and the Successor of Peter, is not some external complement to a particular Church but rather one of its internal constitutive principles, these venerable Christian communities lack something in their condition as particular churches.[17]
On the other hand, because of the division between Christians, the fullness of universality, which is proper to the Church governed by the Successor of Peter and the Bishops in communion with him, is not fully realised in history.[18]
[13] Second Vatican Council, Decree Unitatis redintegratio, 15.3; cf. Congregation for the Doctrine of the Faith, Letter Communionis notio, 17.2: AAS, 85 [1993-II] 848.
[14] Second Vatican Council, Decree Unitatis redintegratio, 14.1.
[15] Cf. Second Vatican Council, Decree Unitatis redintegratio, 14.1; John Paul II, Encyclical Letter Ut unum sint, 56 f: AAS 87 [1995-II] 954 ff.
[16] Second Vatican Council, Decree Unitatis redintegratio, 15.1.
[17] Cf. Congregation for the Doctrine of the Faith, Letter Communionis notio, 17.3: AAS 85 [1993-II] 849.
[18] Ibid.
Thanks.
Eastern, of course, not "Easter".