Constantine the Great began the process that transformed the Roman Empire into a Christian state, as he became a Christian himself. He also pursued social, military and financial reforms to strengthen the empire.
Constantine's father, Constantius, became emperor of the Western Roman Empire in A.D. 305, but he died the next year, leaving his succession in question. Constantine's troops acclaimed him emperor, but he had to fight a series of civil wars before he secured his position in 312. Twelve years later, he conquered the Eastern Roman Empire and ruled as emperor over a unified Roman Empire.
One of Constantine's greatest accomplishments as ruler was his acceptance of Christianity both as on an individual and on a national level. His own devotion to the religion came during the civil wars. Before the Battle of Milvian Bridge, he had a vision telling him to fight in the name of Christ. From this moment on, he remained devoted to the Christian religion. In 313, he issued the Edict of Milan, which legalized Christianity and allowed all religions to worship freely in the Roman Empire. He also convened the Council of Nicaea, a meeting that allowed the great minds of the church to iron out their theological differences and come up with a unified creed.
Constantine also reformed the army, instituting mobile garrisons capable of dealing with internal and external convulsions. To stabilize the economy, he introduced the solidus, a gold coin that remained in use for a thousand years.