Constantine and the Rise of the Church

A bust of Diocletian
In the late Roman Empire, the Christian Church continued to rise in importance. Some historians theorize that the rise of a religion that was not tied to the state diverted the peoples' loyalty away from the emperor and the Roman Empire. Rather than the government leadership, the Church leadership was the moral authority. With different languages, customs, and histories, the people of the Roman Empire around the Mediterranean could unify under the common beliefs of Christianity.
Even with this unification, however, two heresies arose and divided Christians: Donatism and Arianism. Both of these schisms occurred because of discrepancies in belief systems. In the case of Donatism, bishops who had given up their Christian ideals under persecution were told the baptisms they performed were obsolete. Donatism was ultimately rejected. Conversely, Arians believed Christ was the highest of all created beings, but was not one with God. This concept was more widely accepted than Donatism. Eventually, Arianism lost support as well. This marked the time when paganism was no longer the official religion of Rome.
In 284 A.D. (C.E.), the emperor Diocletian came to power and divided the Roman Empire in hopes of better managing it. This division had mixed results, but it did provide the foundation for the differences between an emerging Western Roman Empire and Eastern Roman Empire.
When Constantine came to power in 306 A.D. (C.E.), he too made changes to Rome in hopes of slowing its decline. Constantine moved the capital city to the Greek city of Byzantium, and he renamed it Constantinople. This city was the capital of the Eastern Roman Empire, which transitioned to the Byzantine Empire, until the Ottoman Empire captured it in 1453.
While Diocletian and Constantine did help hold the Roman Empire together for another century, they failed to prevent its fall. In the end, internal and external problems brought the empire down.