Christianity united self-understood "Western Civilization" just as Roman culture had a few centuries earlier. At the same time, because of the peculiarities of Christian belief, it was also a potentially divisive force. Christians spoke a host of different languages and lived across the entire expanse of the Empire. As noted above, there were serious debates around who or what Jesus was. For centuries, there could be no "orthodoxy," meaning "correct belief," because there was no authority within the church (very much including the popes) who could enforce a certain set of beliefs over rival interpretations.
The beginning of orthodoxy was in the second and third centuries, when a group of theologians argued that there were three personas or states of the divine being, referred to as the Holy Trinity. In this view, God could exist simultaneously as three beings: God the Father, the being that spoke in the Old Testament, God the Son, Jesus himself, and God the Holy Spirit, the presence of God throughout the universe. This concept did not quell controversy at all, though, because it created a distinct stance that people could disagree with - rival groups of Christians came to refer to their enemies as "heretics," from the word "heresy," meaning simply "choice."
In the late third century, an Egyptian Christian priest named Arius created a firestorm of controversy when he made a simple logical argument: God the father had created Jesus, so it did not make any sense for Jesus to be the same thing as God. Furthermore, it was impossible to be both human and perfect; since Jesus was human, he was imperfect and could not therefore be God, who was perfect. This belief came to be known as "Arianism" (note that the word has nothing whatsoever to do with the misguided belief in some kind of ancient Germanic race - the "Aryans" - so important to Nazi ideology almost two thousand years later). Arianism quickly took hold among many people, most importantly among the Germanic tribes of the north, where Arian Christian missionaries made major inroads. Thus, Arianism quickly became the largest and most persistent heresy in the early Christian church.
In 325 CE, only a little over a decade after he had converted to Christianity, Constantine assembled a council of church leaders, the Council of Nicaea, to lay Arianism to rest. One of the results was the Nicene Creed (now usually referred to as the Apostles' Creed), to this day one of the central elements of Catholic Mass. In a single passage short enough to commit to memory, the Creed declared belief in Christ’s identity as part of God (“consubstantial to the Father” in its present English translation), Christ’s status as the son of God and the Virgin Mary, Christ’s resurrection, and the promise of Christ’s return at the end of the world. There was now the first “party line” in the early history of Christianity: a specific set of beliefs backed by institutional authority.
While united in belief, the Catholics were divided by language, since the western Empire still spoke Latin and the eastern Empire Greek. In 410 the monk Jerome produced a version of the Christian Bible in Latin, the Vulgate, which was to be the main edition in Europe until the sixteenth century. Surprising from a contemporary perspective, however, is that it was not until 1442 (during the Renaissance) that the definitive and in a sense “final” version of the Bible was established by the Western Church when it defined exactly which books of the Old Testament were to be included and which were not.
Meanwhile, in the east, Greek was not only the language of daily life for many, it was the official language of state in the Empire and the language of the church. The books of the New Testament, starting with the gospels, were written in Greek in the first place, and the Greek intellectual legacy was still very strong. There was an equally strong Jewish intellectual legacy that provided accurate translations from Hebrew and Aramaic to Greek, providing Greek-speaking Christians with access to a reliable version of the Old Testament.
While it certainly clarified the beliefs of the most powerful branch of the institutional church, as the Council of Nicaea defined the official orthodoxy, it guaranteed that there would always be those who rejected that orthodoxy in the name of a different theological interpretation. Likewise, the practical issues of lingual and cultural differences undermined the universalism ("Catholicism") of the Christian church. Those differences and the diversity of belief would only grow over time.