<P> During the reign of the Roman Emperor Constantine the Great ( 306 -- 337 AD ) , Christianity began to transition to the dominant religion of the Roman Empire . Historians remain uncertain about Constantine 's reasons for favoring Christianity , and theologians and historians have often argued about which form of Early Christianity he subscribed to . There is no consensus among scholars as to whether he adopted his mother Helena 's Christianity in his youth , or , as claimed by Eusebius , encouraged her to convert to the faith himself . Some scholars question the extent to which he should be considered a Christian emperor : `` Constantine saw himself as an ' emperor of the Christian people ' . If this made him a Christian is the subject of ... debate , '' although he allegedly received a baptism shortly before his death . </P>
who reigned when christianity was adopted as the official religion of the roman empire
Roman Emperor Constantine the Great