Christianity made the Roman Empire fall because the people who believed this religion won't fight in battles, and spend money on churches instead of roads.
Barbarians made the Roman Empire fall because of their frequent attacks on the western side of the Roman Empire.
One of the reasons about why the Roman Empire fall is because of its lack of money. the gold coins that the Roman Empire made have lesser and lesser gold in them, so people started to charge more for their goods.
Another one of the reasons about why the Roman Empire fall is because that it is too big. It is hard to communicate quickly, and became too hard to govern
It became too hard for the Romans Empire to find enough soldiers to create a army, and most of the people who is in the army is not loyal to the Romans.