The words schism and schismatic are used to denote splits within a church, denomination or religious body. In this context, "schismatic", as a noun, denotes a person who creates or incites schism in a church or a person who is a member of a splinter Church; as an adjective, "schismatic" refers to ideas and activities that are thought to lead to or constitute schism, and ultimately to departure from what the user of the word considers to be the true Christian Church. These words have been used to denote both the phenomenon of Christian group-splintering in general, and certain significant historical splits in particular.

One can make a distinction^[5] between heresy and schism. Heresy is rejection of a doctrine that a Church considered to be essential. Schism is a rejection of communion with the authorities of a Church, and not every break of communion is necessarily about doctrine, as is clear from examples such as the Western Schism and the breaking of the communion that existed between Patriarch Bartholomew I of Constantinople and Archbishop Christodoulos of Athens in 2004.^[6] However, when for any reason people withdraw from communion, two distinct ecclesiastical entities may result, each of which, or at least some members thereof, may then accuse the other(s) of heresy.

In Roman Catholic Church canon law, an act of schism, like an act of apostasy or heresy, automatically brings the penalty of excommunication on the individual who commits it.^[7] As stated in canon 1312 §1 1° of the 1983 Code of Canon Law, this penalty is intended to be medicinal, so as to lead to restoration of unity. Roman Catholic theology considers formal schismatics to be outside the Church, understanding by "formal schismatics" "persons who, knowing the true nature of the Church, have personally and deliberately committed the sin of schism".^[8] The situation, for instance, of those who have been brought up from childhood within a group not in full communion with Rome, but who have orthodox faith, is different: these are considered to be imperfectly, though not fully, related to the Church.^[8] This nuanced view applies especially to the Churches of Eastern Christianity, more particularly still to the Eastern Orthodox Church.^[8] While they don't possess "full communion" (communio in sacris) with the Catholic Church, they are still considered much more linked to it than the Protestant ecclesial communities, which have markedly different theological beliefs and rejected the concept of apostolic succession (with the exception of the Anglicans, which, however, are viewed by the Catholic Church as not having a valid priesthood).

The First Council of Nicaea (A.D. 325) distinguished between schism and heresy. It declared Arian and non-Trinitarian teachings to be heretical and excluded their adherents from the Church. It also addressed the schism between Peter of Alexandria and Meletius of Lycopolis, considering their quarrel to be a matter of discipline, not of faith.

The divisions that came to a head at the Councils of Ephesus (A.D. 431) and Chalcedon (A.D. 451) were seen as matters of heresy, not merely of schism. Thus the Eastern Orthodox Church and Oriental Orthodoxy regard each other as heretical, not orthodox, because of the Oriental Orthodox Church's rejection and the Eastern Orthodox Church's acceptance of the Confession of Chalcedon about the two natures (human and divine) of Christ. However, this view has been challenged in the recent Ecumenical discussion between these two groups, classifying the matter of Chalcedon as a matter of schism, not of heresy.

In its extended and final form (possibly derived from the First Council of Constantinople in 381 although only known from the Acts of the Council of Chalcedon seventy years later),^[9] what is commonly called the Nicene Creed declares belief in the One Holy Catholic and Apostolic Church. Some who accept this creed believe they should be united in a single Church or group of Churches in communion with each other. Others who accept this creed believe it does not speak of a visible organization but of all those baptized who hold the Christian faith, referred to as "Christendom". Some churches consider themselves to be the One Holy Catholic and Apostolic Church. For instance, the Roman Catholic Church claims that title and considers the Eastern Orthodox Church to be in schism, while the Eastern Orthodox Church also claims that title and holds the view that the Catholic Church is schismatic. Some Protestant Churches believe that they also represent the One Holy Catholic and Apostolic Church and consider the Catholic and Orthodox Churches to be in error, while others do not expect a union of all Christian churches on earth. See also One true church and Great Apostasy.

Protestant groups, lacking the stronger traditional authority-structures of (say) Roman Catholicism or Eastern Orthodoxy, and often riven by politico-national divides (sometimes resulting from cuius regio, eius religio), show a high degree of fissibility, which ecumenical efforts may only intensify.^[10]

Schisms have occurred particularly frequently among Anabaptists, to the extent that divisions over even minute details of doctrine and theology are common and scholars have dubbed the phenomenon Täuferkrankheit or "The Anabaptist Disease".^[11] Emphasizing fully voluntary membership in the church, and without an established authority of hierarchical structure, Anabaptists, especially Mennonites have experienced dozens of schisms, resulting in the establishment of dozens of various unaffiliated Mennonite churches.

A current dispute with an acknowledged risk of schism for the Anglican Communion involves responses to homosexuality.

In 2018 Eastern Orthodoxy suffered a schism, the 2018 Moscow-Constantinople schism between the primatial See of Eastern Orthodoxy, the Ecumenical Patriarchate of Constantinople and the Russian Orthodox Church over the issue of Constantinople granting autocephaly to the Orthodox Church of Ukraine.

Christianity

Attachments