The above answer should have noted that Constantine reunited the Eastern and Western Empires as a resulf of the Roman Civil War in which he defeated Maxentius at the Battle of Milvan Bridge. He thus became the sole Emperor thereby eliminating three other claimants. It was at the Battle of Milvan Bridge that he presumably saw the sign of the cross and heard the words In hoc signo vinces (by this sign conquer.) He afterward issued the Edict of Milan which legalized Christianity (but did not make it the official religion of the Empire.) There is some argument that Constantine did so more as a matter of practicality, as Christianity grew within the Empire even though his predecessors had attempted to eliminate it. As for his miraculous vision, Constantine told no one about it until he was on his death bed, at which point he was baptized as a Christian. Whatever his vision or inspiration, it did not stop him from murdering his wife and son. There is some argument that Christian authorities manipulated the story of his conversion to promote the legitimacy of that religion. He did, in fact, authorize the Council of Nicaea which determined that Jesus was both God and man. It also set the date for the celebration of Eastern according to strangely pagan elements. All of his actions, however were dictated by political practicality, not religious conviction.
As noted, Constantine did move the capital to Byzantium and renamed the city for himself (Constantinople) This allowed him to guard the trade routes between East and West.
An excellent work and detailed biography of Constantine is Constantine by Paul Stephenson.
Constantine is one of the most important figures in history because he played a pivotal role within the history of the West. He was the emperor of Rome from 306-337.
Before a battle against Maxentius for the sole rule of Rome, he stated that he saw a sign of the first two letters of Christ in Greek (Chi and Rho) in the sky. He also stated that he heard a voice saying that he would be victorious. He attributed all of this as the Christian God promising him victory. At the battle of the Milvian bridge against Maxentius, he won.
After this victory he converted to Christianity and even saw himself as a pivotal figure in the church. For example, he even called the council of Nicaea, which concerned the deity of Christ.
He also favored the Christian church and this made a huge difference in the history of the world. Through Constantine, we see the rise of the Christian Church. Also he moved the capital of Rome to Constantinople (which was named after him).
Constantine was the founder of the Roman & Byzantine church and was the first to legalize Christianity. To be precise he was a very well known individual because he named an entire city after himself. He created christmas to the liking of the Roman empire by making Christmas on a pagan holiday for winter solstice, yet this is only the beginning. Constantine recieved a vision of a cross in the sun. He told his soldiers to paint crosses on there shields. Jesus told him "In my name conquer." And thus he did, for he sacked the roman cities of that time converting the early byzantines and pagans to Christianity. Constantine is said , to be a great speaker and even a very egotistical man. By legend his mother St.Helena build the church of the Holy seplechur in Jerusalem. Even there the holy rood (the cross of Jesus) is suppose to lye, yet other legends point to it elsewhere as a dark age puzzle. Anyway Constantine was the only christian emperor that built an empire that wasn't in the dark ages. At one time the byzantine empire was full of life and excitment, yet it was till the reign of Justinian the dark came as the black death occured throughout the land. Constantine death is actually quite a mystery as we don't know how he truly died. He was also the man whom created the bible we know today at the council of nicea the bible was constructed into different books. There are even books that where lost that once existed in the biblical cannon and maybe constantine was the key to all of it.