According to Gibbon, the Roman Empire succumbed to barbarian invasions because of a loss of civic virtue among its citizens.They had become weak, outsourcing their duties to defend their Empire to barbarian mercenaries, who then became so numerous and ingrained that they were able to take over the Empire. Romans, he believed, had become effeminate, unwilling to live a tougher, “manly” military lifestyle. In addition, Gibbon argued that Christianity created a belief that a better life existed after death, which fostered an indifference to the present among Roman citizens, thus sapping their desire to sacrifice for the Empire.

So according to The History of the Decline and Fall of the Roman Empire, outsourcing and Christianity led to the fall of the Roman Empire.