From VDH Q&A:
Is there any one event that, had the outcome been different, would have ensured a vigorous Roman Empire for additional centuries?
Hanson: I don’t think a single defeat like an Adrianople was responsible or a dramatic new idea like Christianity made the difference. Instead there was a gradual sense of Balkanization as regional culture, regional languages, and regional politics trumped the central government that no longer was able to unify a diverse empire and convince an affluent and leisured citizenry that Rome was far better than the alternative—a lesson that we ought to keep in mind.
Not So Fast 