Before the "fall" of the Western Roman Empire, there were two major differences between the two halves of the empire. First, the West was a Latin civilization. In that part of the empire, Latin was spoken and Latin/Roman culture dominated. The East was essentially Greek in its language and in its culture. This difference extended to the second main difference, which was religion. The West was dominated by Roman Catholicism. By contrast, the East followed the Orthodox faith.
Distinctions between the two regions became more pronounced after the fall of the Western Empire. But the two parts of the empire were already growing apart even before Rome fell.
The citizens of the empire in Constantinople believed that they were the citizens of Rome. Even after the city of Rome fell in the Fifth Century, contemporary Romans felt that the empire survived by moving East. In this way, Eastern and Western Rome were one in the same. It was only later that historians designated the title Byzantine Empire to the remaining remnants of the Roman Empire in the East. The reason that historians made this distinction is because of some of the cultural differences that exist between the East and West. These differences included variations in religion, language, and architecture between the two cities.
Aside from the obvious language and religious distinctions between Rome and Constantinople, other important cultural differences existed. The Eastern Roman empire was more cosmopolitan in nature than Western Rome and had accepted differing philosophies, religions, and ideas than the city of Rome. While Rome was an important center of trade and commerce, Constantinople was one of a number of important cities in the east. It could be stated the Roman Empire in Constantinople was more of an urban culture than it was when centered in Rome.
Because of the urban nature of the East, it had a much larger population per square mile. The East was also much wealthier than the West. The wealth of the empire was distributed more evenly than the East, which led to less class struggle and more harmony. While Western Rome was rich in territory, much of the land was undeveloped forests. In the East, centers of trade were firmly established and a strong network of trade existed with Asia.
Another important distinction existed in the relationship between the church and the state. Towards the end of the imperial empire in Rome, the Roman Catholic Church exerted considerable authority over the emperors. In the East, the emperors established a system by which the state was firmly in authority over the church and the two entities remained separated. Other differences existed in the legal realm. In Western Rome, laws were diverse in different regions. The Eastern Roman empire established a unified code of law for the entire empire. This allowed for a more unified society. The wealth and trade networks, separation of church and state, and codified system of laws that the Byzantine Empire possessed allowed it to thrive long after Rome fell to barbarian invaders.
The Western portion of the Roman Empire was centered in the city of Rome in Italy in the mid-Mediterranean. The Eastern portion of the Roman Empire was centered in Constantinople, a city built by the Roman Emperor Constantine in 324.
The story of how they were divided begins with Emperor Diocletian who believed the empire was too big to be governed from Rome. He decided to split Rome up into Western and Eastern Rome, each of which would be ruled by co-emperors.
Despite sharing a political system and military, the two portions of the Roman Empire differed culturally. Eastern Rome picked up the Greek language and cultural elements, while Western Rome maintained Latin as a language. Additionally, Eastern Rome split from Roman Catholicism and practiced Orthodox Christianity.
Once the Western Roman Empire fell in 476, the East continued to rule under the Byzantine Empire, which fell to the Ottoman Turks in 1453. Despite the Roman Empire "continuing" until the 15th century, the Eastern portion had such a different culture that it inevitably became a separate entity from its Western Roman counterpart.