After the beginning and spreading of Christianity, which people accepted quickly as the belief of Chritianity was to beleive in one God insted of worshiping different dieties.
Christianity also gave the belief in the after life and hope and courage to people who desperate.
At first Christianity was supressed in early ages, but after Emperor Constantine pronounced Christianity as the states religion, the new religion grew its root in the lower class of society and with time to the whole Empire.
The empire which was said to be unconquereable, and undefeatable because of its huge expensions and its population as well as its governance, and the key factors due to which that mighty empire had to see its decline.
Background and History: The beginning and foundation of Roman empire is a myth.