Eusebius of Caesarea in the context of "Eumelus of Corinth"

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.

John the Presbyter was an obscure figure of the early Christian Church who is either distinguished from or identified with the Apostle John and/or John of Patmos. He appears in fragments from the church father Papias of Hierapolis as one of the author's sources and is first unequivocally distinguished from the Apostle by Eusebius of Caesarea. He is frequently proposed by some as an alternative author of some or all of the Johannine books in the New Testament.

The Ecclesiastical History (Ancient Greek: Ἐκκλησιαστικὴ Ἱστορία, Ekklēsiastikḕ Historía; Latin: Historia Ecclesiastica), also known as The History of the Church and The Church History, is a 4th-century chronological account of the development of Early Christianity from the 1st century to the 4th century, composed by Eusebius, the bishop of Caesarea. It was written in Koine Greek and survives also in Latin, Syriac, and Armenian manuscripts.

The Battle of the Milvian Bridge took place between the Roman Emperors Constantine I and Maxentius on 28 October AD 312. It takes its name from the Milvian Bridge, an important route over the Tiber. Constantine won the battle and started on the path that led him to end the Tetrarchy and become the sole ruler of the Roman Empire. Maxentius drowned in the Tiber during the battle; his body was later taken from the river and decapitated, and his head was paraded through the streets of Rome on the day following the battle before being sent to Africa.

According to Christian chroniclers Eusebius of Caesarea and Lactantius, the battle marked the beginning of Constantine's conversion to Christianity. Eusebius of Caesarea recounts that Constantine and his soldiers had a vision sent by the Christian God. This was interpreted as a promise of victory if the sign of the Chi Rho, the first two letters of Christ's name in Greek, was painted on the soldiers' shields. The Arch of Constantine, erected in celebration of the victory, certainly attributes Constantine's success to divine intervention; however, the monument does not display any overtly Christian symbolism.

The AD 17 Lydia earthquake caused the destruction of at least twelve cities in the region of Lydia in the Roman province of Asia in Asia Minor (now part of Turkey). The earthquake was recorded by the Roman historians Tacitus and Pliny the Elder, and the Greek historians Strabo and Eusebius. Pliny called it "the greatest earthquake in human memory" (Nat. Hist. 2:86 §200). The city of Sardis, the former capital of the Lydian Empire, was the most affected and never completely recovered from the destruction.

Saint Pamphilus (Greek: Πάμφιλος; latter half of the 3rd century – February 16, 309 AD), was a priest of Caesarea and chief among the biblical scholars of his generation. He was the friend and teacher of Eusebius of Caesarea, who recorded details of his career in a three-book Vita that has been lost.