November 8th changed the world.
That was the day Theodosius the First, last emperor of a united Roman Empire) declared Nicene Christianity the official religion of the Roman Empire. Christianity had already become the official religion with the rise of Constantine in 325. It was Constantine that had facilitated the Nicene Council and attempted to organize the previously fractious Christianity. This was a follow up to previous edicts regarding the official state religion. The difference is that up to this point Theodosius had been one of several competing powers in the Empire. 392 saw his consolidation of power and his unquestioned authority.
What it meant:
Theodosius' main targets were the Arians. The Arians taught that Jesus was divine like God but not of the same substance as God the Creator. Nicene Christianity held that Jesus was of the same substance as God. Its a refined argument but it can be grossly simplified like this: Nicene Christianity teaches that Jesus was always God and always existed, Arianism teaches that Jesus was created in a specific point in time. Overall the "Theodosian Decrees" removed all non-Nicene Christians from church office and abolished the last remaining expressions of Roman religion by making its holidays into workdays, banned blood sacrifices, closed Roman temples, outlawing fortune telling and witchcraft, disbanded the Vestal Virgins, and banned the Olympics. During his reign the Oracle of Delphi was destroyed as was the Alexandrian Serapeum, the "daughter" of the famous Library of Alexandria.
This act also gave rise to what would become the Catholic Church and the creation of a single Bible in Latin (begun in 385 while Theodosius was in power).
Essentially the history of Christianity and thus Europe and everything it touched was shaped by Theodosius.
Theodosius death was also the death of Rome. His empire was split between his two sons to eventually become two empires. This in turn lead eventually to the cultural division between Western and Eastern Christianity and thus to the eventual split between the Catholic and Orthodox churches 600 years later.
State Religions, official and otherwise, are always a bad idea.