Theodosius I, also known as Theodosius the Great, was the last Roman Emperor to rule over both the eastern and western halves of the Roman Empire, and he played a crucial role in establishing Christianity as the state religion through the Edict of Thessalonica in 380 AD. His reign marked the final division of the Roman Empire into Eastern and Western entities, which would eventually lead to the fall of the Western Roman Empire and the continuation of the Eastern Roman Empire as the Byzantine Empire.