Emperor Constantine the Great was a Roman emperor who ruled from 306 to 337 AD. He is best known for being the first Roman emperor to convert to Christianity, which significantly influenced the religion's spread throughout the Roman Empire. He also established Constantinople (modern-day Istanbul) as the new capital, transforming it into a major center of trade and culture. Constantine played a crucial role in the Edict of Milan in 313 AD, which granted religious tolerance to Christians and allowed them to practice their faith openly. His reign marked the beginning of a new era in Roman history, as he sought to unify the empire and strengthen its governance.