The eastern half of the Roman empire lasted 1,000 years after the fall of the western half.  Despite it being a part of the roman empire and carrying many new advances in agriculture, learning and technology, it was known by another name and to this day has not received proper representation in history today, "Byzantium".  This is due to the fact that the empire did not just collapse, it was absorbed into the Turkish Ottoman empire. Due to the religious conflict between Christians and Muslims, the Byzantium empire was completely absorbed and was eventually forgotten for the most part. While   the Ottoman empire proudly declared itself a western civilization, and eagerly building off the success of Byzantium there is a century long (albeit misguided) legacy of distinguishing the Byzantium - Ottoman empire from other western civilizations.  Byzantium, it appears cannot be left alone as what remained was eventually renamed Constantinople by Emperor Constantine.

Link to image: https://www.britannica.com/place/Byzantine-Empire

Justian was the most important emperor in early Byzantium, and probably the second most important emperor of all. He was the last emperor to speak Latin as his native language during his reign. it was after His reign that all Roman emperors spoke Greek, which is interesting as Rome and Greece shared similar gods and most historians argue over who came, but in truth I feel like this a chicken or egg situation.  Justian created a tradition that lasted through Byzantium history, the emperor was both the supreme authority in the empire and the spiritual leader of the Christian church. The archbishops of Rome would not succeed in taking power after the empire collapsed for many centuries, In the meantime while he was still alive Justinian launched a campaign to unify all his subjects under Christianity due to his belief as emperor to lead his subjects.  He also desired to wipe out the paganism in Greece and the lands around it.

Link to Image: https://www.thecollector.com/emperor-justinian-byzantine/

When Justian reigned he wanted to unify his people under Christianity and to stomp out any law or tradition that went against his rule. To that end he had the legal experts search through and inform him of any law that needed to be changed or enforced.  His wife Theodora, who came from humble beginnings, was not only a massive help to Justain's reign, but also his other half. Justian and Theodora's marriage was more than political they genuinely loved and valued each other, unlike other political marriages of the time period.  She was also his biggest supporter and confidant as well as helping him with many of the splendid building projects Justian developed and his many social reforms.  Theodora also worked to ensure the end of prostitution, expand the legal rights and protection of women and the protection of children from infanticide.  This is important as seldom few emperors gave their wives this much power or cared about similar issues. This further proves their devotion and love for each other. 

Link to Image:  https://en.wikipedia.org/wiki/Corpus_Juris_Civilis

Both the political and religious unity Justian brought back became undone after he passed away. It's a little sad all that effort and hard work he spent to build his empire vanished after a decade or two.  The religious unity became divided in two between Catholics and Eastern Orthodox. The decline of the empire also steadily approached because of the vast amounts of external threats the empire faced. The empire eventually lost what land it had until it was reduced to the city of Constantinople   and its immediate surrounding territory.  The process was not immediate and took place over many centuries and lasted even longer than the western Roman empire.  This was due to Constantinople's status as one of the largest and strongest cities in the world during the time frame as well as the fact that it had great trade relations with countries and nations as far out as, but not limited to Africa.

Link to image: https://historycollection.com/5-reasons-byzantine-empire-finally-collapsed/