Constantine the Great, ruler of the Roman Empire for 31 years in the early 4th century A.D., was the first Roman emperor to become a Christian convert. He made Christianity legal throughout the empire.
Although he was not yet a Christian at the time, Constantine shaped many aspects of the subsequent Catholic and Greek Orthodox churches by presiding at the First Nicene Council, which decided questions of dogma. He also founded Constantinople, later the center of the Byzantine Empire, as a second Christian center, splitting the empire in a move that may have hastened its downfall. Despite years of work to support Christianity, Constantine was not baptized until shortly before his death in 337 A.D.