Whilst responsibility for administration lay initially solely with the bishop or other regional clergy until the fourth century, from that time onwards higher-level administrative structures developed, modelled on the late antique Roman administrative system: the so-called metropolitan structure and, from the fifth century onwards in particular, the division into patriarchates or legally autonomous, supra-regional church structures. Alongside the patriarchates of Rome, Alexandria, Constantinople, Antioch and Jerusalem, the Church of Cyprus had already emerged during this period as another ‘autocephalous’ church, soon to be followed by the Georgian Church. Autocephalous churches are legally independent and have the right to appoint their own head.

As early as the fifth century, the majority church split into three major church families for ecclesiastical, theological and, ultimately, spiritual reasons. Since the Council of Ephesus in 431, what is now known as the ‘Apostolic Church of the East’ (traditionally, though incorrectly, referred to as ‘Nestorians’) developed; since the Council of Chalcedon in 451, the Oriental Orthodox Churches have emerged (which should correctly be referred to not as ‘Monophysites’, but rather as ‘Miaphysites’ or ‘Non-Chalcedonians’). Whilst the representatives of the ‘Apostolic Church of the East’ built up a vast ecclesiastical structure along the Silk Road as far as China, the Miaphysites include the Copts of Egypt, the Ethiopians and Eritreans, the Syrian Orthodox Church, the Thomas Christians in India and the Armenians. The latter, in particular, were often settled in the immediate vicinity of other ethnic minorities, such as the Germans, as traders and merchants.

Relations between the Patriarchate of Rome and the other autocephalous Churches had already been strained on several occasions in late antiquity. The issuance of the bull of excommunication against Patriarch Michael I Kerullarios (c. 1000–1059), Patriarch of Constantinople from 1043 to 1058, by the Roman legate Humbert of Silva Candida (c. 1010–1061) in 1054 on the altar of Hagia Sophia in Constantinople. Further theological disputes and, in particular, the so-called Fourth Crusade of 1204 sealed the schism. Since then, there have been several attempts to restore ecclesiastical unity. Notable examples include the union talks at Lyon (1279), Ferrara/Florence (1437/1439) and Brest (1596), which were conducted out of political necessity. These union talks gave rise to the so-called United Churches, which celebrate the Orthodox rite but are subject to the Pope in Rome under canon law. Under the Habsburgs, the union of Orthodox Christians with Rome was, from the 18th century onwards, enforced in some cases by force. Consequently, Uniate Churches have been found in Ukraine, Slovakia and Transylvania since that time, and once again following their ban under communist rule since the 1989 revolution.

From the 17th century onwards in particular, a process of denominational formation can also be observed within the Orthodox Churches, for example through the Synod of Jassy/Iaşi (1642). Such a process may have been brought about not only by missionary endeavours on the part of denominations that had emerged in Central and Western Europe, but also by attempts at religious influence on the part of German settlers themselves. However, in the context of the Orthodox Church, one can speak of a territorial policy of Orthodox denominationalisation only in relation to the specific political constellations, particularly within the Ottoman Empire and its peripheral regions, and at most in Russia. Following a process lasting over a century of separation from Constantinople, the Russian Orthodox Church has been an autocephalous patriarchate since 1590. Otherwise, in the early modern period, Orthodox identity was generally linked to a specific ethnic group rather than to a territory, for political reasons. Accordingly, with the emergence of ethnically based nation states in the 19th and early 20th centuries, separate national churches came into being, which were normally initially separated from the Ecumenical Patriarchate of Constantinople. This process remains unfinished to this day. Intense disputes over the ecclesiastical status of Orthodox Christians can thus be observed in Ukraine, the Baltic states and Macedonia, amongst other places.

In addition to the four ancient patriarchates, ten further national churches are recognised by the Orthodox world as a whole, and their autocephaly is undisputed. Alongside these, there are eleven churches with autonomous status. They are administered autonomously but belong to an autocephalous church, from which they receive confirmation upon the election of their head. Some of these autonomous churches are not recognised by all the other Orthodox churches. For example, the Moscow Patriarchate disputes the right of the autonomous Estonian Orthodox Apostolic Church to exist. The Japanese Orthodox Church is not recognised as an autonomous church by the Ecumenical Patriarchate. Even the independence of the autocephalous Orthodox Church in America is a matter of dispute within the wider Orthodox community. The same applies to the Ukrainian Orthodox Church, which is not affiliated with the Moscow Patriarchate.

The only legal institution that ranks above the autocephalous and autonomous churches is the Pan-Orthodox Council. In general, therefore, one can speak of a collegial church structure in which all bishops hold the same rights. Even the patriarchs have no superior jurisdictional authority. Rather, the highest decision-making body within an autocephalous church is the Synod.

Despite their organisational autonomy, Orthodox Churches are characterised by a whole range of common features. These include, to this day, the calculation of the date of Easter according to the Julian calendar. Fests of the fixed calendar, such as Christmas, are celebrated in the individual national churches either according to the Julian calendar (e.g. Russia, Serbia, Mount Athos) or, since 1924, according to the Gregorian calendar (e.g. Greece, Romania). External differences compared with other denominations include, for example, the fact that candidates for the priesthood are permitted to marry before receiving higher orders. Also noteworthy is the direct sequence of baptism, confirmation and first communion for children. Communion is usually celebrated in the form of bread and wine, with the faithful receiving both from a chalice using a spoon. Church life is strongly characterised by monastic-ascetic and mystical elements.

Theologically, the Orthodox Churches reject the so-called ‘filioque ’. This is an insertion into the third part of the early Christian Creeds of Nicaea and Constantinople dating from the year 381. Orthodox Christians view this insertion as implying the subordination of the Holy Spirit to God the Father and the Son. Furthermore, the Orthodox consistently reject, amongst other things, the concept of purgatory and Eucharistic communion with unleavened bread (hosts).

The same applies to the central concepts of the ‘Western’ Enlightenment. For most Orthodox Christians, a strict distinction between transcendence and immanence is incomprehensible. Drawing primarily on Platonic-Neoplatonic conceptions of reality, they instead assume a close connection between the earthly world and the transcendent, which is reflected, amongst other things, in the veneration of icons and relics, as well as in the structure of divine services as the ‘Divine Liturgy’. To explain such connections, the movement known as Palamism developed, in the 14th century, the concept of God’s uncreated energies, through which He acts in the world without, at the same time, being part of that world.

At present, Orthodox Christians are striving to find their place in the modern and postmodern world. In Transylvania, for example, they are therefore developing church-based welfare projects in collaboration with representatives of other denominations. Orthodoxy is making a clear effort to maintain its presence in the media landscape. A clear testimony to the place of Orthodox theology and the Church within contemporary social reality is provided by the so-called ‘Social Doctrine’ of the Russian Orthodox Church from the year 2000, which, amongst other things, sets out the Church’s fundamental ethical principles.

Orthodoxy also drew on the cultural achievements of German settlers: For example, the architects and iconographic programmes of the Orthodox monastery churches in Moldavia, Romania, were influenced by German and Transylvanian-Saxon artists respectively, whilst the monasteries of Mount Athos were supplied with works by goldsmiths who were also active in Transylvania. However, direct contact between Orthodox Christians and Christians of other denominations did not lead solely to constructive encounters. For instance, the Evangelical Lutheran theologian Adolf von Harnack (1851–1930), who grew up in Dorpat/Tartu and therefore had early encounters with Orthodox Christians, developed a negative attitude towards Eastern Orthodox theology and practice. At present, Germans who grew up in Eastern and South-Eastern Europe often play an important and positive role in the bilateral dialogues between German churches and the Orthodox Church.