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. His reign marked a pivotal moment in history, as he established Constantinople (modern-day Istanbul) as the new capital, which became a center of trade and culture. Under his leadership, the Edict of Milan was issued in 313 AD, granting religious tolerance to Christians and ending years of persecution. Constantine's legacy is profound, as he played a crucial role in shaping the future of Europe and the Christian Church.