1. The Great Schism
Religious tensions between the Eastern and Western Roman Empire grew increasing hostile by the mid 700’s AD.
The Eastern Empire had become increasingly pious towards their faith, so much so that in Byzantium, no one could show or possess any religious art/pictures/symbols. The Western Empire “Rome” was ok with the use of religious images and the religious riff between them hit an all time high.
Another growing source of tension was over how each Pope was to be selected. Prior to Justinian all Popes had been picked by the emperor of Rome.
Christianity was also experiencing the effects of false teachings across Asia Minor. The new Arianism contradicted the Roman view to a point that Emperor Constantine called for an official edict that all teachings were to come only from the elected Pope and that Arianism was to be labelled as heresy across the empire. This would become known as the Council of Nicaea.
At Nicaea the notion of “oneness” created the expression “church and state”. Byzantium decreed that this meant the emperor was also the Pope and could set church doctrine.
It also established Rome as the church capital instead of Jerusalem.

2. The Great Schism
In 800 AD, Frankish King Charlemagne protects Rome from a massive barbarian invasion (The Lombards), as a “thank you” Charlemagne is crowned Holy Roman Emperor and for the first time attempts to unite the various European nations under rule of the Pope and himself.
Over the next 300 years, tensions continue to grow between Rome and Constantinople as to who really is the leader of the Christian faith. By AD they are ready to separate not only the leadership but the religion itself.
In 1054, the leaders of both Western and Eastern Roman Churches begin to excommunicate each other and the Schism begins. Over the next few months, the final separation of Rome from Byzantium and Roman Catholic Church from the Eastern Orthodox will take place.
By the end of 1054, there are now officially two separate empires and two separate religions. Those who followed Rome/Western Empire become known as Catholics, those who followed Byzantium/Eastern Empire become known as Orthodox.