It was a little of everything, but starting in the third century AD, the instability of the empire with the military appointing emperors and severe inflation of the currency. In which the military soliders were not paid for protecting and maintaining Rome's borders. The strife between the ruling class (Senate) and imperials created breeding grounds for civil wars hence Constantine was the victor by adopting and politicizing the Christian religion in his cause, and putting end to the persecution of Christians. In the end, the Germanic tribes attacking the borders, weak emperors and no military strength was the downfall of the Roman empire.