The first emperor of the Byzantine Empire was Constantine the Great, who ruled from 306 to 337 AD. Although he was originally a Roman emperor, Constantine significantly influenced the Byzantine Empire's formation during his reign. He is best known for establishing Constantinople (modern-day Istanbul) as the new capital of the Roman Empire in 330 AD, which became a pivotal center for trade, culture, and Christianity. Constantine also played a central role in promoting and legitimizing Christianity within the empire through the Edict of Milan in 313 AD. His reign marked the transition from the Roman Empire to the Byzantine Empire, significantly impacting Western civilization.