Eusebius of Nicomedia in the context of "Apostolic Vicariate of Beirut"

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.

Athanasius I of Alexandria (c. 296–298 – 2 May 373), also called Athanasius the Great, Athanasius the Confessor, or, among Coptic Christians, Athanasius the Apostolic, was a Christian theologian and the 20th patriarch of Alexandria (as Athanasius I). His intermittent episcopacy spanned 45 years (c. 8 June 328 – 2 May 373), of which over 17 encompassed five exiles, when he was replaced on the order of four different Roman emperors. Athanasius was a Church Father, the chief proponent of Trinitarianism against Arianism, and a noted Egyptian Christian leader of the fourth century.

Conflict with Arius and Arianism, as well as with successive Roman emperors, shaped Athanasius' career. In 325, at age 27, Athanasius began his leading role against the Arians as a deacon and assistant to Bishop Alexander of Alexandria during the First Council of Nicaea. Roman Emperor Constantine the Great had convened the council in May–August 325 to address the Arian position that the Son of God, Jesus of Nazareth, is of a distinct substance from the Father. Three years after that council, Athanasius succeeded his mentor as Patriarch of Alexandria. In addition to the conflict with the Arians (including powerful and influential Arian churchmen led by Eusebius of Nicomedia), he struggled against the Emperors Constantine, Constantius II, Julian the Apostate and Valens. He was known as Athanasius Contra Mundum (Latin for 'Athanasius Against the World').