Church History (Eusebius) in the context of "Christianity in the 4th century"

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.

Eusebius of Caesarea (c. AD 260/265 – 30 May AD 339), also known as Eusebius Pamphilius, was a historian of Christianity, exegete, and Christian polemicist from the Roman province of Syria Palaestina. In about AD 314 he became the bishop of Caesarea Maritima.

Together with Pamphilus, Eusebius was a scholar of the biblical canon and is regarded as one of the most learned Christians during late antiquity. He wrote the Demonstrations of the Gospel, Preparations for the Gospel and On Discrepancies between the Gospels, studies of the biblical text. His work Onomasticon is an early geographical lexicon of places in the Holy Land mentioned in the Bible. As "Father of Church History" (not to be confused with the title of Church Father), he produced the Ecclesiastical History, On the Life of Pamphilus, the Chronicle and On the Martyrs. He also produced a biographical work on Constantine the Great, the first Christian Roman emperor, who was Augustus between AD 306 and AD 337.

The persecution in Lyon in AD 177 was an outbreak of persecution of Christians in Lugdunum, Roman Gaul (present-day Lyon, France), during the reign of Marcus Aurelius (r. 161–180), recorded in a contemporary letter preserved in Eusebius's Ecclesiastical History, book 5, chapter 1, which was written 150 years later in Palestine. Gregory of Tours also describes the persecution in the 6th century in De Gloria martyrum.