Polytheism or monotheism

[Antibush5]

You could make an argument that it was the Barbarians destroying the Roman Empire that started the dark ages. Many of these barbaric tribes were not Christian but were converted to it later.

The Roman catholic church opressed most places of seculare learning, they fought most progession every step of the way. Islam would have been a better arguement and only in the period of th Fatimid Calphiate.
also Christianity was the state religion of the Roman Empire in the later half, about 3AD, if you count Constantine as somebody who promoted it as the official religion (a theory I disagree with).