There was a time when Pagans were inclined to blame the Fall of the Roman Empire upon Constantine's decision to adopt Christianity as an official religion. Then a couple of years after Alaric sacked Rome, a Christian named Paulus Orosius wrote a History that argued the position that indeed much of what ailed Rome had arisen prior to the emergence of Christianity, and that on the whole Christianity had done more good than harm. Then, in the eighteenth century when Gibbon wrote his very big book, there was a whole list of possibilities as to why the Roman Empire might of fallen, one of which might indeed have been the empire's adoption of Christianity.
I guess in the western world as it is currently configured it's kind of
difficult to think of the traumatic consequence the Fall of Rome had upon the
psyche of men and women who experienced it. Then for centuries afterwards,
would be kings and emperors looked back to Rome in their attempts to create new
empires and kingdoms. But no one can really answer the question why Rome
disintegrated. The opinions generally flow in the direction of the empire's
internal inability to suitably respond to external threat. A stasis in
expectations that were based upon assumptions that increasingly made no sense.
So in a sense it was failure of idea that did away with the Roman Empire.