In the year 312 Constantine moved into Rome as a winner.

It is told, that a vision of a cross appeared Constantine in the sky. "In this sign you will prevail." ( And not in the sign of the eagle)

Under the reign of the emperor Constantine the Christian faith has been risen to the official religion.  So in West-Rome, in Rome itself, the Catholic Church has been established.

In East-Rome, in Byzantium, arose the Greek Orthodox Church, from which the Russian Orthodox Church originated.

"Byzantium" has been renamed to honor its emperor in "Constantinople", which is now "Istanbul".