Christianity in the form of the Nicene Creed became the official religion of the empire in 380, via the Edict of Thessalonica issued in the name of three emperors – Gratian, Valentinian II, and Theodosius I – with Theodosius clearly the driving force behind it. He was the last emperor of a unified empire: after his death in 395, his sons, Arcadius and Honorius divided the empire into a western and an eastern part. The seat of government in the Western Roman Empire was transferred to Ravenna after the Siege of Milan in 402. During the 5th century, the emperors from the 430s mostly resided in the capital city, Rome.