The correct answer is the spread of Eastern Orthodoxy into Russia
The Byzantine Empire was formed from the division of the Roman Empire in the year 395 into two parts: the Eastern Roman Empire, with capital in Constantinople, and the Western Roman Empire, with capital in Milan.
The city of Constantinople, formerly called Nova Roma, was founded by Constantine in 330, where the Greek colony of Byzantium (now Istanbul) once existed, in the region between Europe and Asia, on the way from the Aegean to the Sea. Black.
Protected by walls and surrounded by water on three sides, the peninsula survived barbarian invasions throughout the Middle Ages.