The first Roman Emperor to convert to Christianity was Constantine I, also known as Constantine the Great. His reign marked a transformative period in Roman history, particularly for the Christian religion. In 313 AD, Constantine issued the Edict of Milan, which granted religious tolerance and ended the persecution of Christians, allowing the religion to flourish within the Roman Empire. His conversion is seen as pivotal because it led to Christianity becoming the dominant religion of the Empire, shaping the cultural, political, and religious landscape of Europe for centuries to come.