Emperor Constantine I, commonly known as Constantine the Great.
The years that Constantine ruled as a Roman emperor during the 4th century CE were extremely important for the history of the Roman Empire. He did not only become a Christian himself, but he is also responsible for the putting in place of several major actions that helped to spread the religion, such as financing the construction of churches and legalizing Christianity, curiously enough he did not become a Christian until he was at his deathbed in the year 337.
Among other crucial things he did, are the reforming of the Roman currency system (he invented a gold coin called "the solid") and the restructuring of Rome Armed forces (he separated civil and military authorities).
Constantinople (now Istanbul, ancient Byzantium) is named after him, and he decided to rebuild it as the city which would serve as the capital of the Empire, even calling it "the second Rome".
