The emperor Theodosius is known as "Theodosius the Great" for several reasons, but his fame rests largely on the fact that his rule was a turning point in history. He was the last Roman Emperor to rule over a united empire. After his death, it split permanently, east and west.
For over a hundred years prior to Theodosius, there had been a continuous tension between the eastern and the western ends of the territory. It split temporarily several times, and was reunited by various leaders. Theodosius worked to create unity, but it did not last.
He faced a number of difficulties: First the Goths, a Germanic tribe from the north, proved beyond any doubt their military superiority to the Romans. Theodosius was forced to allow the Goths to settle inside Roman borders, but they retained their own government in these settlements: a clear weakening of Roman sovereignty.
Secondly, there were civil wars lead by competing Roman politicians; these used up precious resources, and further weakened the credibility of the imperial government.
Thirdly, paganism threatened to make a comeback. Christianity had been growing steadily, but when Theodosius's co-emperor was murdered, suspicion fell onto polytheists as the possible assassins. When a pagan was appointed to replace the fallen co-emperor, it was clear that the polytheists were planning a comeback. Eager to avoid a return to human sacrifice, the ceremonial raping of young virgin girls, and other cruel practices of the pagans, Theodosius began to officially encourage Christianity. He stopped allowing imperial money to be used for pagan ceremonies.
The most powerful contribution to both peace and unity during his reign was Theodosius's support of the Council of Nicaea. This boosting of social harmony would be the Roman Empire's last chance. He died in 395 A.D.