Constantine the Great (about AD274-337), Roman emperor (306-37), the first Roman ruler to be converted to Christianity. He was the founder of Constantinople (present-day Istanbul), which remained the capital of the Eastern Roman (Byzantine) Empire until 1453.
Constantine the Great was born Flavius Valerius Constantinus at Nis, in what is now Serbia, son of the commander Constantius Chlorus (later 
Constantius I) and Helena (later Saint Helena), a camp follower. Constantius became co-emperor in 305. Constantine, who had shown military talent in the East, joined his father in Britain in 306. He was popular with the troops, who proclaimed him emperor when Constantius died later the same year.
Over the next two decades, however, Constantine had to fight his rivals for the throne, and he did not finally establish himself as sole ruler until 324. Following the example of his father and earlier 3rd-century emperors, Constantine in his early life was a solar henotheist, believing that the Roman sun god, Sol, was the visible manifestation of an invisible “Highest God”, who was the principle behind the universe. This god was thought to be the companion of the Roman emperor. Constantine’s adherence to this faith is evident from his claim of having had a vision of the sun god in 310 while in a grove of Apollo in Gaul. In 312, on the eve of a battle against Maxentius, his rival in Italy, Constantine is reported to have dreamed that Christ appeared to him and told him to inscribe the first two letters of his name on the shields of his troops. The next day he is said to have seen a cross superimposed on the sun and the words “in this sign you will be the victor”. Constantine then defeated Maxentius at the Battle of the Milvian Bridge, near Rome. The Senate hailed the victor as savior of the Roman people. Thus, Constantine, who had been a pagan solar worshiper, now looked upon the Christian deity as a bringer of victory. Persecution of the Christians was ended, and Constantine’s co-emperor, Licinius, joined him in issuing the Edict of Milan (313), which mandated toleration of Christians in the Roman Empire. As guardian of Constantine’s favored religion, the church was then given legal rights and large financial donations.
A struggle for power soon began between Licinius and Constantine, from which Constantine emerged in 324 as a victorious Christian champion. Now emperor of both East and West, he began to implement important administrative reforms. The army was reorganized, and the separation of civil and military authority, begun by his predecessor, Diocletian, was completed. The central government was run by Constantine and his council, known as the sacrum consistorium. The Senate was given back the powers that it had lost in the 3rd century, and new gold coins were issued, which remained the standard of exchange until the end of the Byzantine Empire. Constantine intervened in ecclesiastical affairs to achieve unity; he presided over the first ecumenical council of the church at Nicaea in 325. He also began the building of Constantinople in 326 on the site of ancient Greek Byzantium. The city was completed in 330 (later expanded), given Roman institutions, and beautified by ancient Greek works of art. In addition, Constantine built churches in the Holy Land, where his mother (also a Christian) supposedly found the True Cross on which Jesus was crucified. The emperor was baptized shortly before his death, on May 22, 337. Constantine the Great unified a tottering empire, reorganized the Roman state, and set the stage for the final victory of Christianity at the end of the 4th century. Many modern scholars accept the sincerity of his religious conviction.
His conversion was a gradual process; at first he probably associated Christ with the victorious sun god. By the time of the Council of Nicaea (325), however, he was completely Christian, but still tolerated paganism among his subjects. Although criticized by his enemies as a proponent of a crude and false religion, Constantine the Great strengthened the Roman Empire and ensured its survival in the East. As the first emperor to rule in the name of Christ, he was a major figure in the foundation of medieval Christian Europe.
