The Fall of the Roman Empire [ushistory.org]
Constantine and the Rise of Christianity
One of the many factors that contributed to the fall of the Roman Empire was the rise of a new religion, Christianity. The Christian religion, which was monotheistic ran counter to the traditional Roman religion, which was polytheistic (many gods). At different times, the Romans persecuted the Christians because of their beliefs, which were popular among the poor.
This 16th-century medallion depicts Attila the Hun, one of the most vicious invaders of all time.
In 313 C.E., Roman emperor Constantine the Great ended all persecution and declared toleration for Christianity. Later that century, Christianity became the official state religion of the Empire. This drastic change in policy spread this relatively new religion to every corner of the Empire.
By approving Christianity, the Roman state directly undermined its religious traditions. Finally, by this time, Romans considered their emperor a god. But the Christian belief in one god — who was not the emperor — weakened the authority and credibility of the emperor.
Constantine enacted another change that helped accelerate the fall of the Roman Empire. In 330 C.E., he split the empire into two parts: the western half centered in Rome and the eastern half centered in Constantinople, a city he named after himself.