Which Roman emperor is famous for being the first to convert to Christianity, playing a key role in its spread throughout the Roman Empire?

Constantine

Constantine the Great, with his Edict of Milan in 313 AD, not only established religious tolerance for Christianity within the Roman Empire but also laid the foundations for its rise as the dominant religion of the West. His conversion to Christianity and his patronage allowed this faith to flourish, deeply influencing the cultural and political orientation of the empire. Constantine also moved the capital of the empire from Rome to Byzantium, renaming it Constantinople, marking the beginning of a new era for Christianity and the Roman Empire. His reign was therefore a pivot in the history of Christianity, transforming it from a persecuted sect into a state religion, thus shaping the religious landscape of Europe for millennia to come.