For the Catholic Church and the Eastern Orthodox Church, full communion exists only between Christians who form a single church. Protestants understand full communion as instead a matter of practical relations among denominations that nonetheless fully retain their distinct identities.
The Catholic Church makes a distinction between full and partial communion. Where full communion exists, there is but one Church. Partial communion, on the other hand, exists where some elements of Christian faith are held in common, but complete unity on essentials is lacking. For instance, the Roman Catholic Church sees itself as in partial communion with Protestants, and as in much closer, but still incomplete, communion with the Orthodox Churches.
It has expressed this idea in many documents. The Decree on Ecumenism of the Second Vatican Council, 3 states: "... quite large communities came to be separated from full communion with the Catholic Church ... men who believe in Christ and have been truly baptized are in communion with the Catholic Church even though this communion is imperfect". The Catechism of the Catholic Church, citing documents of the Second Vatican Council and of Pope Paul VI, states:
Full communion involves completeness of "those bonds of communion - faith, sacraments and pastoral governance - that permit the Faithful to receive the life of grace within the Church."http://clsa.org/content/files/USCCB_memo_2006_0405.pdf
The particular Churches that form the Catholic Church are each seen, not as a separate body that has entered into practical arrangements concerning its relations with the others, but as the embodiment in a particular region or culture of the one Catholic Church.
The 28 May 1992 Letter to the Bishops of the Catholic Church on some aspects of the Church understood as Communion expressed this idea as follows::
The universal Church cannot be conceived as the sum of the particular Churches, or as a federation of particular Churches. It is not the result of the communion of the Churches, but, in its essential mystery, it is a reality ontologically and temporally prior to every individual particular Church.
The autonomous Catholic Churches in full communion with the Holy See are:
The Catholic Church sees itself as in partial, not full communion, with other Christian groups. "With the Orthodox Churches, this communion is so profound that it lacks little to attain the fullness that would permit a common celebration of the Lord's Eucharist" Catechism of the Catholic Church (838).
In fact, full communion is seen as an essential condition for sharing together in the Eucharist, apart from exceptional circumstances, in line with the second-century practice witnessed to by Saint Justin Martyr, who, in his First Apology http://www.ccel.org/fathers/ANF-01/just/justinapology1.html#Section66, wrote: "No one is allowed to partake (of the Eucharist) but the man who believes that the things which we teach are true, and who has been washed with the washing that is for the remission of sins, and unto regeneration, and who is so living as Christ has enjoined."
Accordingly, "Catholic priests are forbidden to concelebrate the Eucharist with priests or ministers of Churches or ecclesial communities which do not have full communion with the Catholic Church."
The Directory for the Application of Principles and Norms on Ecumenism, 122 - 136 indicates the circumstances in which some sharing in sacramental life, especially the Eucharist, is permitted with other Christians.
The norms there indicated for the giving of the Eucharist to other Christians are summarized incanon 844 of the Code of Canon Law as follows:
§3. Catholic ministers administer the sacraments of penance, Eucharist, and anointing of the sick licitly to members of Eastern Churches which do not have full communion with the Catholic Church if they seek such on their own accord and are properly disposed. This is also valid for members of other Churches which in the judgment of the Apostolic See are in the same condition in regard to the sacraments as these Eastern Churches.
§4. If the danger of death is present or if, in the judgement of the diocesan bishop or conference of bishops, some other grave necessity urges it, Catholic ministers administer these same sacraments licitly also to other Christians not having full communion with the Catholic Church, who cannot approach a minister of their own community and who seek such on their own accord, provided that they manifest Catholic faith in respect to these sacraments and are properly disposed.
See also: List of Orthodox Churches.
Eastern Orthodox and Oriental Orthodox Christians have an understanding of what full communion means that is very similar to that of the Catholic Church. Though they have no figure corresponding to that of the Roman Catholic Pope, performing a function like that of the Pope's Petrine Office, they see each of their autocephalous Churches as embodiments of, respectively, the one Eastern Orthodox Church or the one Oriental Orthodox Church. They too consider full communion an essential condition for common sharing in the Eucharist. The Ecumenical Patriarch of Constantinople, as first among equals among the Eastern Orthodox autocephalous churches and their spiritual leader, though not having authority similar to that of the Roman Catholic Pope, serves as their spokesman. The Pope of the Coptic Orthodox Church of Alexandria holds a somewhat similar position in Oriental Orthodoxy.
For the autocephalous Churches that form the Eastern Orthodox Church, see Eastern Orthodox Church organization. Their number is somewhat in dispute.
Other churches see full communion between them as meaning that their members may licitly participate in each others' rites, particularly in the partaking of the Eucharist in closed communion denominations, and involving also recognition of each other's offices of ministry as valid and thus, in most cases, interchangeability of ordained ministers. Importantly, the existence of full communion, as thus understood, does not presume that there is no difference in rites or in doctrine between the two Churches, but rather that these differences do not touch on points defined as essential.
The word "intercommunion" is sometimes used of this arrangement, which is much less close than the unity between Churches that share a common history, such as the Anglican Communion.
This understanding of "full communion" differs from that of the Roman Catholic Church and Eastern Christianity in that the churches that enter into such arrangements do not consider themselves as forming together a single church.
It is in the stronger sense of becoming a single church that the Traditional Anglican Communion sought "full communion" with the Roman Catholic Church as a sui iuris (particular Church) jurisdiction. Its membership is now deciding whether to accept the offer of full communion (again in the stronger sense) within the framework of personal ordinariates of the Latin Rite of the Roman Catholic Church.
The following groupings of churches have arrangements for or are working on arrangements for: