State church of the Roman Empire in the context of "First seven ecumenical councils"

A state religion (also called official religion) is a religion or creed officially endorsed by a sovereign state. A state with an official religion (also known as a confessional state), while not a secular state, is not necessarily a theocracy. State religions are subject to advantageous treatment by official or government-sanctioned establishments of them, ranging from incentivising citizens to recognise and practice them through government endorsement to having public spending on the maintenance of religious property and clergy be unrestricted, but the state does not need to be under the legislative control of the clergy as it would be in a theocracy. Generally, these religions have more rights and fewer restrictions in the country than other religions in a country.

Official religions have been known throughout human history in almost all types of cultures, reaching into the Ancient Near East and prehistory. The relation of religious cult and the state was discussed by the ancient Latin scholar Marcus Terentius Varro, under the term of theologia civilis (lit. 'civic theology'). The first state-sponsored Christian denomination was the Armenian Apostolic Church, established in 301 CE. In Christianity, as the term church is typically applied to a place of worship for Christians or organizations incorporating such ones, the term state church is associated with Christianity as sanctioned by the government, historically the state church of the Roman Empire in the last centuries of the Empire's existence, and is sometimes used to denote a specific modern national branch of Christianity. Closely related to state churches are ecclesiae, which are similar but carry a more minor connotation.

The Church Fathers, Early Church Fathers, Christian Fathers, or Fathers of the Church were ancient and influential Christian theologians and writers who established the intellectual and doctrinal foundations of Christianity. The historical period in which they worked became known as the Patristic Era and spans approximately from the late 1st to mid-8th centuries, flourishing in particular during the 4th and 5th centuries, when Christianity was in the process of establishing itself as the state church of the Roman Empire.

For many denominations of Christianity, the writings of the Ante-Nicene Fathers, Nicene Fathers and Post-Nicene Fathers are included in Sacred Tradition. As such, in traditional dogmatic theology, authors considered Church Fathers are treated as authoritative for the establishment of doctrine. The academic field of patristics, the study of the Church Fathers, has extended the scope of the term, and there is no definitive list. Some, such as Origen and Tertullian, made major contributions to the development of later Christian theology, but certain elements of their teaching were later condemned.

During the reign of the Roman emperor Constantine the Great (306–337 AD), Christianity began to transition to the dominant religion of the Roman Empire. Historians remain uncertain about Constantine's reasons for favoring Christianity, and theologians and historians have often argued about which form of early Christianity he subscribed to. There is no consensus among scholars as to whether he adopted his mother Helena's Christianity in his youth, or, as claimed by Eusebius of Caesarea, encouraged her to convert to the faith he had adopted.

Constantine ruled the Roman Empire as sole emperor for much of his reign. Some scholars allege that his main objective was to gain unanimous approval and submission to his authority from all classes, and therefore he chose Christianity to conduct his political propaganda, believing that it was the most appropriate religion that could fit with the imperial cult. Regardless, under the Constantinian dynasty Christianity expanded throughout the empire, launching the era of the state church of the Roman Empire. Whether Constantine sincerely converted to Christianity or remained loyal to paganism is a matter of debate among historians. His formal conversion in 312 is almost universally acknowledged among historians, despite that it was claimed he was baptized only on his deathbed by the Arian bishop Eusebius of Nicomedia in 337; the real reasons behind it remain unknown and are debated also.

The Edict of Milan (Latin: Edictum Mediolanense; Greek: Διάταγμα τῶν Μεδιολάνων, Diatagma tōn Mediolanōn) was the February 313 agreement to treat Christians benevolently within the Roman Empire. Western Roman Emperor Constantine I and Emperor Licinius, who controlled the Balkans, met in Mediolanum (modern-day Milan) and, among other things, agreed to change policies towards Christians following the edict of toleration issued by Emperor Galerius two years earlier in Serdica. The Edict of Milan gave Christianity legal status and a reprieve from persecution but did not make it the state church of the Roman Empire, which occurred in AD 380 with the Edict of Thessalonica, when Nicene Christianity received normative status.

The document is found in Lactantius's De mortibus persecutorum and in Eusebius of Caesarea's History of the Church with marked divergences between the two. Whether or not there was a formal 'Edict of Milan'  is no longer really debated among scholars, who generally reject the story as it has come down in church history.

In the 5th century in Christianity, there were many developments which led to further fracturing of the State church of the Roman Empire. Emperor Theodosius II called two synods in Ephesus, one in 431 and one in 449, that addressed the teachings of Patriarch of Constantinople Nestorius and similar teachings. Nestorius had taught that Christ's divine and human nature were distinct persons, and hence Mary was the mother of Christ but not the mother of God. The Council rejected Nestorius' view causing many churches, centered on the School of Edessa, to a Nestorian break with the imperial church. Persecuted within the Roman Empire, many Nestorians fled to Persia and joined the Sassanid Church (the future Church of the East) thereby making it a center of Nestorianism. By the end of the 5th century, the global Christian population was estimated at 10-11 million. In 451 the Council of Chalcedon was held to clarify the issue further. The council ultimately stated that Christ's divine and human nature were separate but both part of a single entity, a viewpoint rejected by many churches who called themselves miaphysites. The resulting schism created a communion of churches, including the Armenian, Syrian, and Egyptian churches, that is today known as Oriental Orthodoxy. In spite of these schisms, however, the imperial church still came to represent the majority of Christians within the Roman Empire.

At the end of the 4th century the Roman Empire had effectively split into two states although its economy and the Church were still strongly tied. The two halves of the empire had always had cultural differences, in particular exemplified by the widespread use of the Greek language in the Eastern Empire and the more limited use of Greek in the West (Greek was used in the West but Latin was displacing it as the spoken vernacular. By the 5th century scholars in the West had begun to abandon Greek in favor of the use of Latin. The Church in Rome, in particular, began to encourage the use of Latin in the western provinces and published Jerome's Vulgate, the first authorized translation of the Bible in Latin.

Christianity in the 4th century was dominated in its early stage by Constantine the Great and the First Council of Nicaea of 325, which was the beginning of the period of the First seven Ecumenical Councils (325–787), and in its late stage by the Edict of Thessalonica of 380, which made Nicene Christianity the state church of the Roman Empire.