Final answer:
The first Roman Emperor to become a Christian and help prevent the persecution of Christians was Constantine. He enacted the Edict of Milan to legalize Christianity and made significant contributions to the Christian religion, including moving the capital to Constantinople.
Step-by-step explanation:
The Roman Emperor who was the first to become a Christian and played a role in preventing the persecution of Christians was Constantine. Constantine had a profound impact on the Roman Empire's attitude toward Christianity through legislative changes, most notably the Edict of Milan in 313, which decreed religious toleration and made Christianity legal within the empire. After a victorious battle, which he attributed to a vision sent by the Christian God, Constantine supported Christian institutions and empowered Christian officials, thus significantly expanding the presence of Christianity in the empire. He also established Constantinople as the new capital of the Roman Empire, further entrenching the influence of Christianity.
Among the options provided, it is clear that some emperors played different roles relative to Christianity. Nero was known for the persecution of Christians, Augustus reigned before the emergence of Christianity, and Justinian ruled much later and continued the tradition of an emperor exerting influence over the church. However, it was under Constantine's rule that Christianity began its ascension within the empire's social and political hierarchy.