The influential figure behind the establishment of the Byzantine Empire is Emperor Constantine the Great. He reigned from 306 to 337 AD and is credited with founding Constantinople, which became the capital of the Byzantine Empire. His conversion to Christianity also played a pivotal role in shaping the culture and religious landscape of the empire. Constantine's leadership not only transformed the Roman Empire but also ensured the continuity of the Eastern Roman Empire, which thrived for over a millennium after the fall of its western counterpart.