Diocletianic Persecution in the context of "St. Erasmus"

Flavius Valerius Constantius (c. 250 – 25 July 306), also called Constantius I, was a Roman emperor from 305 to 306. He was one of the four original members of the Tetrarchy established by Diocletian, first serving as caesar from 293 to 305 and then ruling as augustus until his death. Constantius was also father of Constantine the Great, the first Christian emperor of Rome. The nickname "Chlorus" (Ancient Greek: Χλωρός, lit. 'the Pale') was first popularized by Byzantine-era historians and not used during the emperor's lifetime.

As an Illyrian soldier of humble origin, Constantius had a distinguished military career and rose to the top ranks of the army. Around 289, he set aside Helena, Constantine's mother, to marry a daughter of Emperor Maximian, and in 293 was added to the imperial college by Maximian's colleague Diocletian. Assigned to rule Gaul, Constantius defeated the usurper Carausius there and his successor Allectus in Britain, and campaigned extensively along the Rhine frontier, defeating the Alamanni and Franks. When the Diocletianic Persecution was announced in 303, Constantius ordered the demolition of churches but did not actively hunt down Christians in his domain. Upon becoming senior emperor in May 305, Constantius launched a successful punitive campaign against the Picts beyond the Antonine Wall. He died suddenly at Eboracum (York) in July the following year.

Galerius Valerius Maximianus (/ɡəˈlɛəriəs/; Greek: Γαλέριος; c. 258 – May 311) was Roman emperor from 305 to 311. He participated in the system of government later known as the Tetrarchy, first acting as caesar under Emperor Diocletian. In this period Galerius obtained victory warring against the Persian Sassanian Empire, defeating Narseh at the battle of Satala in 298 and possibly sacking the Sassanian capital of Ctesiphon in 299. He also campaigned across the Danube against the Carpi, defeating them in 297 and 300. Galerius was promoted to augustus upon the abdication of Diocletian in 305, but had to contend with multiple usurpers as the Tetrarchic system broke down. Although he was a staunch opponent of Christianity, he ended the Diocletianic Persecution by issuing the Edict of Serdica in 311.

In the year before the First Council of Constantinople in 381, Nicene Christianity became the official religion of the Roman Empire when Theodosius I, emperor of the East, Gratian, emperor of the West, and Gratian's junior co-ruler Valentinian II issued the Edict of Thessalonica in 380, which recognized the catholic orthodoxy, as defined by the Council of Nicea, as the Roman Empire's state religion. Historians refer to this state-sponsored church using a variety of terms: the catholic church, the orthodox church, the imperial church, the Roman church, or the Byzantine church, with some also used for wider communions extending beyond the Roman Empire. The Eastern Orthodox Church, Oriental Orthodoxy, and the Catholic Church all claim to stand in continuity from the Nicene Christian church to which Theodosius granted recognition. Political differences between the Eastern Roman Empire and the Persian Sassanid Empire led to the separation of the Church of the East in 424. A doctrinal split within the imperial church led to the independence of early Oriental Orthodoxy, while the fall of the Western Roman Empire initiated the gradual separation between Eastern and Western Christianity, culminating in the East-West schism of 1054. The Western Church evolved into the Latin Catholic Church while the Eastern Church that remained under the patronage of the Eastern empire and evolved into the Greek Orthodox Church.

Earlier in the 4th century, following the Diocletianic Persecution of 303–313 and the Donatist controversy that arose in consequence, Constantine the Great had convened councils of bishops to define the orthodoxy of the Christian faith and to expand on earlier Christian councils. A series of ecumenical councils convened by successive Roman emperors met during the 4th and the 5th centuries, but Christianity continued to suffer rifts and schisms surrounding the theological and christological doctrines of Arianism, Nestorianism, Miaphysitism, and Dyophysitism. In the 5th century, the Western Roman Empire decayed as a polity; invaders sacked Rome in 410 and in 455, and Odoacer, an Arian barbarian warlord, forced Romulus Augustus, the last nominal Western Emperor, to abdicate in 476. However, apart from the aforementioned schisms, the church as an institution persisted in communion, if not without tension, between the East and West. In the 6th century, the Byzantine armies of the Byzantine Emperor Justinian I recovered Italy and other regions of the Western Mediterranean shore. The Byzantine Empire soon lost most of these gains, but it held Rome, as part of the Exarchate of Ravenna, until 751, a period known in church history as the Byzantine Papacy. The early Muslim conquests of the 7th–9th centuries would begin a process of converting most of the then-Christian world in the Levant, Middle East, North Africa, regions of Southern Italy and the Iberian Peninsula to Islam, severely restricting the reach both of the Byzantine Empire and of its church. Christian missionary activity directed from the capital of Constantinople did not lead to a lasting expansion of the formal link between the church and the Byzantine emperor, since areas outside the Byzantine Empire's political and military control set up their own distinct churches, as in the case of Bulgaria in 919.

The Era of the Martyrs (Latin: anno martyrum), also known as the Diocletian era (Latin: anno Diocletiani), is a method of numbering years based on the reign of Roman Emperor Diocletian who instigated the last major persecution against Christians in the Empire. It was used by the Church of Alexandria beginning in the 4th century AD and it has been used by the Coptic Orthodox Church of Alexandria from the 5th century until the present. This era was used to number the year in Easter tables produced by the Church of Alexandria.

Diocletian began his reign on 20 November 284 AD, and the reference epoch (day one of the Diocletian era) was assigned to be the first day of that Alexandrian year, 1 Thoth, the Egyptian New Year, or 29 August 284 AD.

The military saints, warrior saints and soldier saints are patron saints, martyrs and other saints associated with the military. They were originally composed of the early Christians who were soldiers in the Roman army during the persecution of Christians, especially the Diocletianic Persecution of AD 303–313.

Most of the early Christian military saints were soldiers of the Roman Empire who had become Christian and, after refusing to participate in Imperial cult rituals of loyalty to the Roman Emperor, were subjected to corporal punishment including torture and martyrdom.

Donatism was a schism from the Catholic Church in the Archdiocese of Carthage from the fourth to the sixth centuries. Donatists argued that Christian clergy must be faultless for their ministry to be effective and their prayers and sacraments to be valid. Donatism had its roots in the long-established Christian community of the Roman province Africa Proconsularis (present-day Tunisia, the northeast of Algeria, and the western coast of Libya) and Mauretania Tingitana (roughly with the northern part of present-day Morocco), in the persecutions of Christians under Diocletian. Named after the Berber Christian bishop Donatus Magnus, Donatism flourished during the fourth and fifth centuries. Donatism mainly spread among the indigenous Berber population, and Donatists were able to blend Christianity with many of the Berber local customs.

The Edict of Serdica, also called Edict of Toleration by Galerius, was issued in 311 in Serdica (now Sofia, Bulgaria) by Roman Emperor Galerius. It officially ended the Diocletianic Persecution of Christianity in the Eastern Roman Empire.

The Edict implicitly granted Christianity the status of religio licita, a worship that was recognized and accepted by the Roman Empire. It was the first edict legalizing Christianity and preceded the Edict of Milan by two years.