Happy birthday to Theodosius I, Roman emperor. Theodosius was known as "the Great". There are various reasons for bestowing such an epithet but the most common is conquering a lot of shit. Theo I did indeed conquer quite a lot. Mostly what he conquered was bits of the Roman empire. It has to be stated that the empire was in a bit of a mess when he took the throne. The empire was divided for administrative regions into two halves each with its own emperor. The two rulers were supposed to cooperate but spent most of their time arguing with each other. In addition the eastern empire (the bit Theodosius took over) had come down with a bad case of Goths.
The Goths had requested asylum in the empire due to the activities of the Huns on their borders but once there the imperial administrators ripped them off so thoroughly that they rose in revolt. The emperor of the east, one Valens by name mustered a large army and marched against them. The army was slaughtered and Valens himself was killed. The western emperor, Gratian, invited Theodosius to take over what was left of the eastern army and as such he became de facto emperor. The next couple of years were awkward ones with Theodosius attempting to fight the Goths without having much of an army (Gratian gave up and went home). Finally he managed to come to an arrangement whereby the Goths would serve in his army (considering the casualties the Roman army had taken by this point a better term would be; to serve as his army) in return for which the Goths could do pretty much what they liked.
With peace theoretically established Theodosius settled down to rule those bits of his empire not overrun by Goths. Over in the west Gratian had been killed and Theodosius backed his replacement against a series of usurpers. Now the Goths came into their own and in a bunch of civil wars Theodosius was victorious while simultaneously reducing his Gothic problem by making sure they were always in the front line of his battles. After the death of the emperor in the west (officially he committed suicide according to his highly ambitious military commander) Theodosius crushed a couple more usurpers and settled down to rule the whole empire. He was the last emperor to rule the entire empire singlehandedly.
Possibly that last fact is why he was called "the Great". Certainly he was an improvement on his predecessor. Another reason is because of his strong backing of the Christian religion (and Christians were writing most of the histories by this stage). Unlike previous Christian emperors who had espoused their religion while quietly letting followers of other gods get on with their lives Theodosius was an intolerant suppressor of both paganism and heresy. The church loved him, popular opinion was divided.
Theodosius might have had cause to regret this after an unfortunate massacre in Thessalonica. His garrison there was mainly Goths and they treated the population rather badly. The people protested and in the course of the protest the Gothic commander was killed. On hearing this Theodosius lost his temper and demanded vengeance. The Goths (who were just waiting for the word) slaughtered several thousand people (messages from the emperor countermanding his original instructions were conveniently delayed). It was a public relations disaster. Shortly thereafter Theodosius toddled off to church in Milan which is where he was staying at the time. The bishop of Milan was a chap named Ambrose and he wasn't a guy to mess with. As the emperor approached Ambrose barred him from the church until he did appropriate penance for the deaths ordered in his name. Theodosius was effectively excommunicated and not until he appeared on his knees in sackcloth in front of the church begging God's forgiveness did Ambrose let him inside. It was the first time a churchman had invoked the authority of God as superior to that of the emperor and the first time an emperor acknowledged that he had a boss.
Another possible reason for Theodosius being called "the Great" is a comparison with his successors. He appointed his sons Arcadius and Honorius as emperors in the east and the west respectively to rule after he was gone. Both of them turned out to be worthless idiots. With those two as his successors and Valens as his predecessor all Theodosius really had to do to gain the epithet "the Great" was breath through his nose and tie his own sandals.