It is always best to put things in writing. There seems to be more permanence in doing so compared to an oral history or a verbal agreement, and one way to hold leaders to account is by putting their laws and decrees into written form.
As the Roman Empire was fading into oblivion at the Byzantine Era was on the rise, there became a new need in codifying Roman law in the Empire in the fifth century A.D., as there was no legal record of the now-Christian Empire that was established a century earlier by Emperor Constantine.
Theodosius II appointed nearly two dozen lawyers and politicians to compile a codex, or set of laws or decrees, established in the Roman Empire from the time of Constantine’s reign in the early part of the fourth century A.D. forward to Theodosius’ tenure as head of the Empire. The special committee first got together in 429 and spent nearly nine full years in gathering all of the legislation that was passed and/or decreed by the emperors during the previous 100-plus years, known as the early Christian era in the Empire.
Finally, in 438, the codex was unveiled and it became the legal standard of society for more than 600 years, carrying into the Middle Ages. Written in Latin (the dominant language of Greece, the eastern part of the Roman Empire that eventually became part of the Byzantine Empire), the codex was furnished to the Senate in Rome, as well as to officials in Constantinople (present-day Istanbul, Turkey) for consideration across the entire Roman Empire.
There were two main items that made the Codex Theodosianus so vital in history. First of all, this was the main codex of laws that essentially guided the Byzantine Empire – which was the part of the Roman Empire that survived the fall (the eastern half of the Empire, containing Greece and much of the current-day Middle East) and lasted until the Renaissance in Europe – and carried forward into the Middle Ages in Europe as the “common law” of the time.
The second key part about the codex is that it is known for codifying Christianity in the latter stages of the Roman Empire and for most of the Byzantine Empire. Christianity was not only encouraged in the Empire, but the Codex Theodosianus was the first known text that made Christianity an official religion of a region, and it even went so far as to foment a reputation of Christian intolerance by making all other religions illegal in the Empire.
In other words, Christianity in some ways taking the brunt today of what the Roman/Byzantine Empire decided about Christianity. Christianity itself wasn’t intolerant; the government that created a Christian theocracy developed the intolerance in favor of Christianity. This alleged intolerance by Christians was essentially brought about by the Theodosian Code, and some aspects of that code lasted through the Middle Ages and stretched from the Middle East and Greece eventually to western Europe.
The Codex Theodosianus, for all intents and purposes, codified the expansion of the burgeoning religion of Christianity, which had only started to take hold in the previous century following the establishment of the Holy Bible by the Council of Nicaea (of while Emperor Constantine had a vital role, being an early evangelical). Making Christianity not only the official religion of the Roman and Byzantine empires, but the only religion to be observed, established the foundation for Christianity to become a major worldwide religion for centuries to come.