Nicene Christianity in the context of "State church of the Roman Empire"

Jewish Christians were the followers of a Jewish religious sect that emerged in Roman Judea during the late Second Temple period, under the Herodian tetrarchy (1st century AD). These Jews believed that Jesus was the prophesied Messiah and they continued their adherence to Jewish law. Jewish Christianity is the historical foundation of Early Christianity, which later developed into Nicene Christianity (which comprises the Roman Catholic, Eastern Orthodox, Oriental Orthodox, and Protestant traditions) and other Christian denominations.

Christianity started with Jewish eschatological expectations, and it developed into the worship of Jesus as the result of his earthly ministry in Galilee and Jerusalem, his crucifixion, and the post-resurrection experiences of his followers. Jewish Christians drifted apart from Second Temple Judaism, and their form of Judaism eventually became a minority strand within mainstream Judaism, as it had almost disappeared by the 5th century AD. Jewish–Christian gospels are lost except for fragments, so there is a considerable amount of uncertainty about the scriptures which were used by this group of Christians.

Theodosius I (Ancient Greek: Θεοδόσιος Theodosios; 11 January 347 – 17 January 395), also known as Theodosius the Great, was Roman emperor from 379 to 395. He won two civil wars and was instrumental in establishing the Nicene Creed as the orthodox doctrine for Nicene Christianity. Theodosius was the last emperor to rule the entire Roman Empire before its administration was permanently split between the Western Roman Empire and the Eastern Roman Empire. He ended the Gothic War (376–382), but did so on terms disadvantageous to the empire, with the Goths remaining and politically autonomous within Roman territory, albeit as nominal allies.

Born in Hispania, Theodosius was the son of a high-ranking general of the same name, Theodosius the Elder, under whose guidance he rose through the ranks of the Roman army. Theodosius held independent command in Moesia in 374, where he had some success against the invading Sarmatians. Not long afterwards, he was forced into retirement, and his father was executed under obscure circumstances. Theodosius soon regained his position following a series of intrigues and executions at Emperor Gratian's court. In 379, after the eastern Roman emperor Valens was killed at the Battle of Adrianople against the Goths, Gratian appointed Theodosius as a successor with orders to take charge of the military emergency. The new emperor's resources and depleted armies were not sufficient to drive the invaders out; in 382 the Goths were allowed to settle south of the Danube as autonomous allies of the empire. In 386, Theodosius signed a treaty with the Sasanian Empire which partitioned the long-disputed Kingdom of Armenia and secured a durable peace between the two powers.

The Oriental Orthodox Churches are Eastern Christian churches adhering to Miaphysite Christology, with approximately 60 million members worldwide. However, the Oriental Orthodox Churches, individually, claim approximately 87 million baptized members. The Oriental Orthodox Churches adhere to the Nicene Christian tradition. Oriental Orthodoxy is one of the oldest branches in Christianity.

As some of the oldest religious institutions in the world, the Oriental Orthodox Churches have played a prominent role in the history and culture of countries and regions such as Armenia, Egypt, Eritrea, Ethiopia, Sudan, the Levant, Iraq and the Malabar region of southern India. As autocephalous churches, their bishops are equal by virtue of episcopal ordination. Their doctrines recognize only the first three ecumenical councils as valid.

In Christian theology, the incarnation is the belief that the pre-existent divine person of Jesus Christ, God the Son, the second person of the Trinity, who is also the Logos (Koine Greek for 'word'), was "made flesh" by being conceived through the power of the Holy Spirit in the womb of a woman, the Virgin Mary, who is also known as the Theotokos (Greek for "God-bearer" or "Mother of God"). The doctrine of the incarnation then entails that Jesus was at the same time both fully God and fully human.

In the incarnation, as traditionally defined by those Churches that adhere to the Council of Chalcedon, the divine nature of the Son was united but not mixed with human nature in one divine person, Jesus, or according to those adhering to the Council of Ephesus, the divine and human natures of Christ are fully united into one composite nature "without mixing, confusion, or separation". This is central to the traditional faith held by most Christians. Alternative views on the subject (see Ebionites and the Gospel of the Hebrews) have been proposed throughout the centuries, but all were rejected by Nicene Christianity.

The Nicene Creed, also called the Creed of Constantinople, is the defining statement of belief of Nicene Christianity and in those Christian denominations that adhere to it.

The original Nicene Creed was first adopted at the First Council of Nicaea in 325. According to the traditional view, forwarded by the Council of Chalcedon of 451, the Creed was amended in 381 by the First Council of Constantinople as "consonant to the holy and great Synod of Nice." However, many scholars comment on these ancient Councils, saying "there is a failure of evidence" for this position since no one between the years of 381–451 thought of it in this light. Further, a creed "almost identical in form" was used as early as 374 by St. Epiphanius of Salamis. Nonetheless, the amended form is presently referred to as the Nicene Creed or the Niceno-Constantinopolitan Creed.

The Holy Spirit, otherwise known as the Holy Ghost, is a concept within the Abrahamic religions. In Judaism, the Holy Spirit is understood as the divine quality or force of God manifesting in the world, particularly in acts of prophecy, creation and guidance. In Nicene Christianity, this conception expanded in meaning to represent the third person of the Trinity, co-equal and co-eternal with God the Father and God the Son. In Islam, the Holy Spirit acts as an agent of divine action or communication. In the Baha’i Faith, the Holy Spirit is seen as the intermediary between God and man and "the outpouring grace of God and the effulgent rays that emanate from His Manifestation".

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.

Gratian (/ˈɡreɪʃiən/; Latin: Gratianus; 18 April 359 – 25 August 383) was emperor of the Western Roman Empire from 367 to 383. The eldest son of Valentinian I, Gratian was raised to the rank of Augustus as a child and inherited the West after his father's death in 375. He nominally shared the government with his infant half-brother Valentinian II, who was also acclaimed emperor in Pannonia on Valentinian's death. The East was ruled by his uncle Valens, who was later succeeded by Theodosius I.

Gratian subsequently led a campaign across the Rhine, attacked the Lentienses, and forced the tribe to surrender. That same year, the eastern emperor Valens was killed fighting the Goths at the Battle of Adrianople, which led to Gratian elevating Theodosius to replace him in 379. Gratian favoured Nicene Christianity over traditional Roman religion, issuing the Edict of Thessalonica, refusing the office of pontifex maximus, and removing the Altar of Victory from the Roman Senate's Curia Julia. The city of Cularo on the Isère river in Roman Gaul was renamed Gratianopolis after him, which later evolved to Grenoble. By 383 Gratian had become unpopular with his army, which abandoned him during a confrontation with the usurper Magnus Maximus near Lutetia (Paris). Gratian fled to Lugdunum and was later murdered.