Rome Becomes Christian, Western Empire Ends
Though Constantine I (circa A.D. 280 to 337) was the first Christian emperor of Rome, it was not until decades after he died that Christianity became the official religion of the empire.
Ruling from 379 to 395, Emperor Theodosis I was the first emperor to decree that all citizens should be Christian. He was also the last ruler to preside over a united Eastern and Western Roman Empire. Indeed, east and west had been diverging since long before his reign.
Theodosis allowed most pagans to worship in peace. But he forcibly removed "heretic" Arian or Gnostic bishops from Christian churches throughout the eastern empire.
These officials were replaced with "catholic" Christians, as defined by the circa-325 Council of Nicea. The creed recognized the equal divinity of the holy Trinity: Father, Son, and Holy Spirit.
After Theodosis' death in 395 the empire split into western and eastern halves, with the eastern half being much stronger.
The Western Roman Empire, including much of Italy, Spain, Gaul, Britain, and North Africa, was plagued by economic stagnation and invaders such as the Visigoths.
By A.D. 402 the Western Empire was unable to protect even its ancient capital from hordes of barbarian invaders. The emperor Honorius moved the capital from Rome to Ravenna, the stronghold port of the empire's Adriatic Sea fleet. Rome's run at the head of the empire was unequivocally over.
The waning western empire was governed from Ravenna until its final end in 476, when Germanic tribesmen overthrew Emperor Romulus Augustulus.