Emperor Constantine the Great was a Roman ruler who reigned 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 the city of Constantinople, which later became the capital of the Byzantine Empire. 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 transition of the Roman Empire from paganism to Christianity, shaping the future of Europe and the Western world.