Christianity did not contribute to the fall of the western part
of the Roman Empire. Some pagan Romans believed that it weakened
the Romans by weakening their fighting spirit. This was untrue and
this view reflected their dislike for and bias against
Christianity. The Christian emperors of the later Empire were as
resolute in trying to defend the Roman Empire as the previous pagan
emperors had been.
The co-emperors Gratian and, especially, Theodosius I (or the
Great) aimed at religious uniformity. Their Edict of Thessalonica
(380) made mainstream Christianity the sole legitimate religion of
the empire and banned dissident Christian doctrines, which were
branded heretic. The main target was Arian Christianity which was
the main dissident Christian doctrine and was popular around the
empire. Theodosius immediately started persecuting the Arians. He
also persecuted Roman religion and other pagan creeds, particularly
Manichaeism which was briefly the main rival to Christianity in the
competition to replace classical paganism.
Theodosius promoted greater religious uniformity in the Roman
Empire and, through this, greater cohesion. The western part of the
Roman Empire fell under the weight of the invasions by the Germanic
peoples. The Germanic invaders were also Christians even though
they had been converted to the Arian form of Christianity, The
eastern part of the Roman Empire, which was just as Christian as
the western part, was not affected by these invasions and continued
to exist for nearly 1,000 years.
Mainstream Christianity was 'Western or Latin Christianity' and
'Eastern or Greek Christianity.' The former was the main form of
Christianity in the western part of the Roman Empire and the latter
the main form in the eastern part. Later they came to be called
Catholic and Orthodox respectively.